trash, scum (noun)
a film of impurities or vegetation that can form on the surface of a liquid
remove the scum from
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
The topmost liquid layer of a cesspool or septic tank.
A reprehensible person or persons.
To remove the layer of scum from (a liquid etc.).
To remove (something) as scum.
To scour (the land, sea etc.).