cotton rush vs cotton grass

cotton rush and cotton grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
cotton rush	Yes	No	No	No
cotton grass	Yes	No	No	No

As nouns, cotton rush and cotton grass are synonyms defined as:

cotton rush and cotton grass: any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes

cotton rush (noun)	cotton grass (noun)
any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes	any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes