Scum

1. To form a scum; to become covered with scum. Also used figuratively.
2. refuse; recrement; anything vile or worthless.
3. 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.
4. To sweep or range over the surface of.
5. To take the scum from; to clear off the impure matter from the surface of; to skim.