salad greens vs celtuce

salad greens and celtuce both are nouns.

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

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

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

salad greens (noun)	celtuce (noun)
greens suitable for eating uncooked as in salads	leaves having celery-like stems eaten raw or cooked
	lettuce valued especially for its edible stems