salad greens vs greens

salad greens and greens both are nouns.

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

As nouns, greens is a hypernym of salad greens; that is, greens is a word with a broader meaning than salad greens:

salad greens: greens suitable for eating uncooked as in salads
greens: any of various leafy plants or their leaves and stems eaten as vegetables

Other hypernyms of salad greens include green, leafy vegetable.

salad greens (noun)	greens (noun)
greens suitable for eating uncooked as in salads	any of various leafy plants or their leaves and stems eaten as vegetables