a film of impurities or vegetation that can form on the surface of a liquid
"After leaving the bucket in the sun all day, a thick layer of green scum floated on top of the stagnant water."
A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
"The foreman ordered the workers to skim off the scum from the top of the molten iron before pouring it into the molds."
remove the scum from
"She carefully skimmed the white foam off the surface of the boiling broth to prevent it from clogging the pot's drain."
To remove the layer of scum from (a liquid etc.).
"She carefully skimmed the fat off the top of the soup before serving it."