Scums

noun

1. A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).

   "During smelting, scum rises to the surface and is then removed by the smelter."

2. A greenish water vegetation (such as algae), usually found floating on the surface of ponds

   "These organisms form scum in large quantities."

3. The topmost liquid layer of a cesspool or septic tank.
4. (chiefly US) semen
5. A reprehensible person or persons.

   "People who sell used-up pens are scum, just total low-lives."