scum vs dross

scum and dross both are nouns.

Word	Noun	Adjective	Verb	Adverb
scum	Yes	No	Yes	No
dross	Yes	No	No	No

As nouns, dross is a hyponym of scum; that is, dross is a word with a more specific, narrower meaning than scum:

scum: a film of impurities or vegetation that can form on the surface of a liquid
dross: the scum formed by oxidation at the surface of molten metals

Other hyponyms of scum include scoria, slag.

scum (noun)	dross (noun)
a film of impurities or vegetation that can form on the surface of a liquid	the scum formed by oxidation at the surface of molten metals
worthless people	worthless or dangerous material that should be removed

scum (verb)	dross (verb)
remove the scum from