a film of impurities or vegetation that can form on the surface of a liquid
source: WordNet 3.0
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.
source: Webster 1913
verb
remove the scum from
source: WordNet 3.0
To take the scum from; to clear off the impure matter from the surface of; to skim.
source: Webster 1913
To form a scum; to become covered with scum. Also used figuratively.
