mustard vs mustard greens

mustard and mustard greens both are nouns.

Word	Noun	Adjective	Verb	Adverb
mustard	Yes	No	No	No
mustard greens	Yes	No	No	No

As nouns, mustard and mustard greens are synonyms defined as:

Other synonyms of mustard include Indian mustard, leaf mustard.

mustard (noun)	mustard greens (noun)
pungent powder or paste prepared from ground mustard seeds	leaves eaten as cooked greens
leaves eaten as cooked greens
any of several cruciferous plants of the genus Brassica