surplus vs superfluous

surplus is a noun but superfluous is not a noun.

Word	Noun	Adjective	Verb	Adverb
surplus	Yes	Yes	No	No
superfluous	No	Yes	No	No

As adjectives, surplus and superfluous are synonyms defined as:

Other synonyms of surplus include excess, extra, redundant, spare, supererogatory, supernumerary.

surplus (noun)	superfluous (noun)
a quantity much larger than is needed

surplus (adjective)	superfluous (adjective)
more than is needed, desired, or required	more than is needed, desired, or required
	serving no useful purpose; having no excuse for being