Which color algae is typically yellow and wall-clinging in pools?

• A. Green algae
• B. Blue-green algae
• C. Yellow algae
• D. Black algae

Answer: (C)

Identifying algae by color and where it grows helps you know how stubborn it will be to remove. Yellow algae, often called mustard algae, typically appears as a yellow or tan film on pool walls and tends to cling tightly to vertical surfaces. Its adhesive, root-like structures anchor to plaster, tile, or grout, which makes it more difficult to scrub away than some other types. You’ll commonly see it in areas with limited circulation or after periods of low sanitizer, especially on shady walls or behind ladders. Because of its color and wall-clinging habit, yellow algae fits this description best, distinguishing it from green algae (usually greener and easier to brush off), blue-green algae (cyanobacteria, more of a surface issue), and black algae (dark growth that embeds in cracks).