salad green vs leafy vegetable

salad green and leafy vegetable both are nouns.

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

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

salad green: 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 green include green, greens.

salad green (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