mountain rice vs rice grass

mountain rice and rice grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain rice	Yes	No	No	No
rice grass	Yes	No	No	No

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

mountain rice: valuable forage grass of dry upland areas and plains of western North America to northern Mexico
rice grass: any grass of the genus Oryzopsis

Other hypernyms of mountain rice include ricegrass.

mountain rice (noun)	rice grass (noun)
valuable forage grass of dry upland areas and plains of western North America to northern Mexico	any grass of the genus Oryzopsis