salad greens vs cress

salad greens and cress both are nouns.

Word	Noun	Adjective	Verb	Adverb
salad greens	Yes	No	No	No
cress	Yes	No	No	No

As nouns, cress is a hyponym of salad greens; that is, cress is a word with a more specific, narrower meaning than salad greens:

Other hyponyms of salad greens include lettuce, celtuce, chicory, curly endive, chicory escarole, endive, escarole, salad burnet.

salad greens (noun)	cress (noun)
greens suitable for eating uncooked as in salads	pungent leaves of any of numerous cruciferous herbs
	any of various plants of the family Cruciferae with edible leaves that have a pungent taste