Green algae
Free-floating, turns the whole pool cloudy then green, and is the easiest to kill. It means chlorine was too low relative to CYA, filtration was inadequate, or both.
Fix: correct CYA if it is too high, hold a proper chlorine level continuously for several days, brush daily, and run the filter around the clock. Chemistry first, then patience.
Mustard algae — the Central Texas special
Yellow-brown, dusty, clings to shaded walls, steps, and behind ladders, and brushes away in a cloud that resettles in hours.
It tolerates chlorine levels that would kill green algae, which is why owners think it 'keeps coming back' when it never actually died.
It also lives on everything that went in the water: pool toys, floats, brushes, poles, swimsuits, and the cleaner. Treating the water but not the equipment restarts the cycle.
Fix: elevated chlorine held longer than feels necessary, aggressive brushing of every shaded surface, cleaning or replacing the filter media, and sanitizing every piece of gear.
Black algae
Dark spots with a protective outer layer, rooted into plaster, usually in rough or porous spots. It is technically cyanobacteria and it is the hardest to remove.
Fix: break the heads open with a stiff brush so chlorine can reach the roots, treat, and repeat. Rough plaster is its habitat, so a surface in poor condition will keep hosting it.
Preventing the repeat bloom
Hold chlorine relative to your CYA, not to a fixed number. This is the single biggest factor.
Brush the shaded and dead-flow areas weekly, not just the visible floor.
Keep phosphates from running away in a pool with heavy organics or fill-water contribution — it is not a substitute for chlorine, but it removes fuel.
Verify circulation reaches the corners. A dead spot is an algae nursery no chemical fixes.
