leaf beet vs greens

leaf beet and greens both are nouns.

Word	Noun	Adjective	Verb	Adverb
leaf beet	Yes	No	No	No
greens	Yes	No	No	No

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

leaf beet: long succulent whitish stalks with large green leaves
greens: any of various leafy plants or their leaves and stems eaten as vegetables

Other hypernyms of leaf beet include green, leafy vegetable.

leaf beet (noun)	greens (noun)
long succulent whitish stalks with large green leaves	any of various leafy plants or their leaves and stems eaten as vegetables
beet lacking swollen root; grown as a vegetable for its edible leaves and stalks