Pool problem (5)
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" .)