a film of impurities or vegetation that can form on the surface of a liquid(noun.artifact)
source: wordnet30
The extraneous matter or impurities which rise to the surface of liquids in boiling or fermentation, or which form on the surface by other means; also, the scoria of metals in a molten state; dross.(noun)
source: webster1913
verb (v)
remove the scum from(verb.possession)
source: wordnet30
To take the scum from; to clear off the impure matter from the surface of; to skim.(verb)
source: webster1913
To form a scum; to become covered with scum. Also used figuratively.(verb)