scum vs slag

scum and slag both are nouns.

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

As nouns, slag is a hyponym of scum; that is, slag 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
slag: the scum formed by oxidation at the surface of molten metals

Other hyponyms of scum include dross, scoria.

scum (noun)	slag (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

scum (verb)	slag (verb)
remove the scum from	convert into slag