People say "my pool has algae" as if it is one problem. It is not. The green cloud that shows up after a storm and the stubborn dark spots rooted into your plaster are different organisms with different defenses, and treating them the same way is why owners shock a pool three times and stay dirty. Our general guide to preventing and clearing algae covers the shared protocol. This post goes a layer deeper on identification, because the type you have determines how hard the fight is going to be.
There are three you will actually run into in the Triad, and they run from easy to genuinely stubborn.
Green algae: the common, easy one
This is the one most people mean. Green algae is free-floating and it turns the water hazy, then a cloudy green, then pea soup if you ignore it. It can also cling to walls and steps as a slimy green film. It is the fastest to appear — a hot weekend with the chlorine dipped, or a big storm dumping organics into the pool, and it can bloom overnight.
The saving grace is that green algae is the easiest to kill. It has no meaningful defense against chlorine. Balance your pH, brush the surfaces to break up any film, shock the pool to a genuinely elevated chlorine level, run the pump continuously, and hold that level until the water turns from green to cloudy gray-white (that color shift is dead algae) and then to clear. Most green blooms clear in a day or two of disciplined treatment. If yours does not, either your chlorine is not actually reaching the water because of high stabilizer, or you are not dealing with green algae at all.
Yellow / mustard algae: the one that fools you
Yellow algae, often called mustard algae, is the middle child and the deceptive one. It shows up as a dusty, yellowish-tan powder on the shady side of the pool — the walls and steps that do not get direct sun, behind ladders, in corners the return jets miss. The trap is that it brushes off easily and the water looks clean, so owners think they beat it. Then it is back the next day in the same spot, because brushing only knocked it loose, it did not kill it.
Mustard algae is notably more chlorine-resistant than green, and it colonizes surfaces rather than floating in the water column. Killing it takes higher chlorine held for longer, and it takes decontaminating everything that touched the water, because mustard algae hides on your gear. Brushes, poles, floats, ladders, the skimmer, even swimsuits and towels can hold it and reinfect the pool the moment you think you are done. Brush all the surfaces, shock and hold a high chlorine level for several days, and run your equipment and toys through the treated water so nothing reseeds it. Miss the pole and you start over.
Black algae: the hardest fight in the pool
Black algae is the one that ruins weekends. It is technically a cyanobacteria, and it shows up as dark blue-green to black spots, usually dime to quarter sized, that look almost painted onto the surface. Two things make it brutal. First, it grows a tough protective outer layer that ordinary chlorine simply cannot penetrate. Second, it does not sit on the surface — it sends roots down into the pores of plaster and into grout lines, so even if you kill the top, the roots regrow the spot.
You cannot chemical your way through black algae. The only thing that works is mechanical plus chemical, in that order:
- Break the cap. Scrub each spot hard with a stiff stainless steel brush, but only on plaster or concrete — never stainless on vinyl or fiberglass, which it will destroy. You are physically tearing off the protective layer so chlorine can reach the living cells underneath.
- Hit the exposed heads directly. Apply chlorine straight onto the freshly scrubbed spots while they are vulnerable. Some people break a chlorine tab and rub it right on the spot for this.
- Shock and hold. Raise the whole pool to a high chlorine level and keep it there, with the pump running continuously.
- Repeat over multiple days. Black algae does not die in one pass. Brush and treat the same spots day after day until they stop coming back.
Why aging plaster harbors black algae
Here is the part that matters for the long term. Black algae needs somewhere to root, and a smooth, sound surface gives it very little to grab. A rough, etched, aging plaster surface gives it everything — pores, pits, and micro-cracks where roots dig in and where your brush cannot fully reach. That is why black algae so often shows up in older pools, and why it keeps coming back in the same spots no matter how disciplined you are. If you are fighting chronic black algae in a plaster surface that scrapes your feet and stains, the surface itself is the real problem. No amount of chlorine fixes a substrate that is built to harbor the stuff, and the durable fix is a resurface to a smooth, sound finish that gives algae nothing to root into.
Prevention beats all three
Every one of these organisms is easier to prevent than to kill, and prevention comes down to the same three habits regardless of type:
- Circulation. Algae starts in dead spots where water does not move. Make sure your return jets are aimed to sweep the whole pool and that your pump and filter are actually turning the water over. Weak or short circulation runs is an open invitation, and it is often the real reason algae keeps starting in the same shady corner.
- Chemistry. Keep free chlorine in range every day, not on average, and keep your stabilizer under control so that chlorine actually works. Most "my chlorine tests fine but I have algae" problems trace back to water chemistry that looks right and sanitizes wrong.
- Brushing. Brush the walls and floor regularly, especially the shady spots and steps. It breaks up films before they establish and it keeps mustard and black algae from getting a foothold.
Our humidity and warm water speed all of it up
The Triad hands algae ideal conditions for a good chunk of the year. Warm water is where algae thrives, and our long, humid, stormy summers keep pool water in the algae comfort zone from late spring well into fall. Add the pollen and organic load of a Piedmont spring and the heavy thunderstorms that dump debris and knock your chemistry sideways, and you have a climate that grows algae fast whenever your sanitizer slips. That is not a reason to panic — it is a reason to stay ahead of it with steady chemistry and runtime instead of reacting after the bloom.
A word on safety
Algae treatment means handling concentrated chemicals, so read the label on everything, every time. Never mix products, especially different chlorine types or chlorine with acid, because the fumes are genuinely dangerous. Add chemicals to water, never water to chemicals, dose one thing at a time with the pump running, and store everything dry and separated. The label is the instruction manual, and it is written the way it is for good reason.
If you are fighting the same black or mustard algae every summer no matter what you do, the pool is telling you something about its surface or its circulation. Call (336) 471-0103 or request a consultation, and we will find out why it keeps coming back instead of selling you another bucket.