Crossword Genius

Pool problem (5)

Ross

I believe the answer is:

algae

'pool problem' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for algae that I've seen before include "Primitive organisms" , "Seaweeds etc" , "Simple organisms, e.g. seaweeds" , "Type of seaweed" , "Water plants with no true stems or leaves" .)

I've seen this clue in the Penny Press and The Wall Street Journal.
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