salad green vs greens

salad green and greens both are nouns.

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

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

salad green: 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 green include green, leafy vegetable.

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