Soapy residue (4)

I believe the answer is:

scum

(Other definitions for scum that I've seen before include "Impurities on surfaces of liquid" , "Dirty surface of a liquid" , "Impurities on surface of liquid - or bad people" , "Waste matter floating on the surface" , "Floating film of filth" .)

I've seen this clue in the Newsday.com and the Newsday.

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