manna grass vs sweet grass

manna grass and sweet grass both are nouns.

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

As nouns, manna grass and sweet grass are synonyms defined as:

manna grass and sweet grass: any of several moisture-loving grasses of the genus Glyceria having sweet flavor or odor

manna grass (noun)	sweet grass (noun)
any of several moisture-loving grasses of the genus Glyceria having sweet flavor or odor	any of several moisture-loving grasses of the genus Glyceria having sweet flavor or odor