cereal vs rice grass

cereal and rice grass both are nouns.

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

As nouns, rice grass is a hyponym of cereal; that is, rice grass is a word with a more specific, narrower meaning than cereal:

cereal: grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet
rice grass: any grass of the genus Oryzopsis

cereal (noun)	rice grass (noun)
a breakfast food prepared from grain	any grass of the genus Oryzopsis
foodstuff prepared from the starchy grains of cereal grasses
grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet

cereal (adjective)	rice grass (adjective)
made of grain or relating to grain or the plants that produce it