sweet grass vs manna grass

sweet grass and manna grass both are nouns.

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

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

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

sweet grass (noun)	manna 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