Which description matches yellow algae in pools?

• A. Free-floating algae
• B. Grows only in sunlight
• C. Can be killed by brushing
• D. Wall clinging found on shady side of pool; treated with chlorine; may require replacing cartridge, sand, grids or it could return

Answer: (D)

Yellow algae that clings to walls is the wall-attaching mustard-type growth that tends to appear in shady pool areas. It doesn’t drift freely like some algae; instead, it sticks to surfaces and can hide in niches where light is scarce. Treating it requires chlorine, typically a shock dose, and thorough scrubbing of the affected surfaces. Because this type of algae can persist in filter media, you may also need to inspect and potentially replace parts of the filtration system (cartridges, sand, or grids) to prevent it from returning. The other descriptions don’t fit because free-floating growth isn’t characteristic of this algae, it doesn’t require sunlight to thrive—shade actually helps it—brushing alone isn’t enough without proper chlorine treatment, and the real issue often lies in its ability to linger in the filter rather than just on walls.