Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Polluted water can form a frothy green scum on the surface. This "pond scum" is actually a living organism, algae. The algae are plant-like protists. Usually the algae are too small to notice, but sometimes algae grow in excess because of the excess nutrients in polluted water. Answer: algae
What organism constitutes frothy green "pond scum"?