scum

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.”

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

“These organisms form scum in large quantities.”

The topmost liquid layer of a cesspool or septic tank.

(chiefly US) semen

A reprehensible person or persons.

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

To remove the layer of scum from (a liquid etc.).

To remove (something) as scum.

To become covered with scum.

To scour (the land, sea etc.).

To gather together, as scum.

To startscum or savescum.