salad greens vs leafy vegetable

salad greens and leafy vegetable both are nouns.

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

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

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

Other hypernyms of salad greens include green, greens.

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