manna grass vs meadow grass

manna grass and meadow grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
manna grass	Yes	No	No	No
meadow grass	Yes	No	No	No

As nouns, meadow grass is a hypernym of manna grass; that is, meadow grass is a word with a broader meaning than manna grass:

manna grass: any of several moisture-loving grasses of the genus Glyceria having sweet flavor or odor
meadow grass: any of various grasses that thrive in the presence of abundant moisture

Other hypernyms of manna grass include meadowgrass.

manna grass (noun)	meadow grass (noun)
any of several moisture-loving grasses of the genus Glyceria having sweet flavor or odor	any of various grasses that thrive in the presence of abundant moisture