noun

scum

SKUM/skʌm/

An unwanted film of residue at a surface.

A thin layer of greenish scum floats on the pond.

A dirty thin greenish film floats on stagnant pond water.

Etymology

From “foam, froth,” scum narrowed to impurities rising on liquid and then a film of dirt. That association with filth supplied the contemptuous label for people.

In context

Max the Meat Guy12:39

I feel like I just took a scoop of scum from the bottom of a swamp and shoved it in my mouth. Pond scum.

I Cooked EVERY Animal’s Brisket

More examples

Soap scum builds up on the shower walls.
She skims the scum off the simmering broth.
A ring of scum marks the waterline inside the bucket.

Forms

scum

scummy adjective — Covered with an unpleasant surface film.

Similar words

film: Any thin surface layer, not necessarily dirty or unwanted.
residue: Material left behind, not necessarily forming a surface film.
froth: A layer of bubbles, not necessarily containing dirt or impurities.