Telling mustard algae apart
Mustard algae is yellow to brownish and powdery, and it tends to wipe off easily, which fools people into thinking it is just dirt or pollen. The tell is that it returns to the same spots after you brush it away, because it clings more tightly than dust and resists normal chlorine.
Because it is more chlorine-tolerant than green algae, a regular shock often knocks it back without finishing it, and it returns within days. That is why the approach has to be more aggressive and more thorough.
Shock higher than usual
Brush all the affected surfaces first to lift the algae, then raise free chlorine to your shock level and hold it. Mustard algae is stubborn, so keep the chlorine at shock level and brush daily, running the pump continuously, until it stops returning. The shock calculator sets the target from your CYA, and keeping pH in range with the pH calculator makes the chlorine more effective.
Do not declare victory the moment it looks clear. Mustard algae routinely comes back a day or two later if you stop early, so hold the level until it is genuinely gone.
Clean everything that touched the water
This is the step people skip, and it is why mustard algae keeps coming back. It survives on anything that went in the pool: swimsuits, towels, pool toys, floats, the brush, the vacuum, and the skimmer and pump baskets. If you do not disinfect or wash those, they reinfect the water as soon as chlorine drops.
While the pool is at shock level, throw swimsuits and removable items into the water to disinfect them, wash what you can on a hot cycle, and rinse equipment. Clean the filter thoroughly during and after, since it traps the algae.
Keep it from returning
Once clear, the prevention is steady chlorine and good circulation, the same as any algae. Keep free chlorine in range for your CYA every day with the chlorine calculator, brush shady low-flow areas regularly, and do not let chlorine drift low in hot weather, which is when mustard algae makes its comeback.
Frequently asked questions
How do I get rid of mustard algae in my pool?
Brush all affected surfaces, raise chlorine to your shock level, and hold it there with daily brushing and the pump running until the algae stops returning. Crucially, also disinfect everything that touched the water, swimsuits, toys, brushes, and equipment, since mustard algae survives on them and reinfects the pool.
Why does mustard algae keep coming back?
Two reasons: it tolerates chlorine better than green algae, so a normal shock often does not finish it, and it survives on swimsuits, toys, brushes, and equipment that reinfect the water once chlorine drops. Holding shock level longer and disinfecting everything that touched the water is what stops the cycle.
Is mustard algae the same as green algae?
No. Mustard or yellow algae is a different, more chlorine-tolerant type that looks like fine sand or pollen and clings to shady surfaces. Green algae blooms in the water and clears more readily with a standard shock. Mustard algae needs a higher, sustained chlorine level and a thorough cleanup of items.
How is mustard algae different from sand or pollen?
It looks similar, yellow and powdery, but pollen and dirt vacuum or filter out and stay gone, while mustard algae returns to the same spots after you brush it away. That return is the tell that it is algae and needs chlorine treatment, not just cleaning.
What chlorine level kills mustard algae?
Raise free chlorine to the shock level for your cyanuric acid and hold it there, brushing daily, until it stops returning. Because mustard algae is more chlorine-tolerant than green algae, you keep the level up longer rather than dosing once. Use the shock calculator to set the target from your CYA.