scum

n. 1 layer of dirt, froth, etc. At the top of liquid. 2 derog. Worst part, person, or group (scum of the earth). v. (-mm-) 1 remove scum from. 2 form a scum (on).  scummy adj. (-ier, -iest). [low german or dutch]

v. 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.