Aquarium buildup
I believe the answer is:
scum
'aquarium buildup' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scum that I've seen before include "Impurities on surface of liquid - or bad people" , "Impure film" , "Undesirable people" , "Floating impurities or worthless people" , "Impurities on liquid's surface" .)