skim vs skim off

skim is a noun but skim off is not a noun.

Word	Noun	Adjective	Verb	Adverb
skim	Yes	Yes	Yes	No
skim off	No	No	Yes	No

As verbs, skim and skim off are synonyms defined as:

Other synonyms of skim include cream, cream off.

skim (noun)	skim off (noun)
reading or glancing through quickly
a thin layer covering the surface of a liquid

skim (adjective)	skim off (adjective)
used of milk and milk products from which the cream has been removed

skim (verb)	skim off (verb)
read superficially	pick the best
move or pass swiftly and lightly over the surface of	remove from the surface
remove from the surface
cause to skip over a surface
coat (a liquid) with a layer
travel on the surface of water
examine hastily