SCUM

noun

1. A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
2. A greenish water vegetation (such as algae), usually found floating on the surface of ponds
3. The topmost liquid layer of a cesspool or septic tank.

verb

1. To remove the layer of scum from (a liquid etc.).
2. To remove (something) as scum.
3. To become covered with scum.