To become covered with or as if with scum

ETYM Of Scand. origin; cf. Dan. and Swed. skum, Icel. skűm, lg. schum, Dutch schuim, Old High Germ. scűm, German schaum; probably from a root meaning, to cover. Related to Hide skin, Meerschaum, Skim, Sky.
A film of impurities or vegetation that can form on the surface of a liquid.