Semen or Cowper's fluid.
Semen.
A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
A greenish water vegetation (such as algae), usually found floating on the surface of ponds
A reprehensible person or persons.
The topmost liquid layer of a cesspool or septic tank.
To remove (something) as scum.
To startscum or savescum.
To remove the layer of scum from (a liquid etc.).
To become covered with scum.