Agrostis alba vs meadow grass

Agrostis alba and meadow grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
Agrostis alba	Yes	No	No	No
meadow grass	Yes	No	No	No

As nouns, meadow grass is a hypernym of Agrostis alba; that is, meadow grass is a word with a broader meaning than Agrostis alba:

Agrostis alba: slender European grass of shady places; grown also in northeastern America and temperate Asia
meadow grass: any of various grasses that thrive in the presence of abundant moisture

Other hypernyms of Agrostis alba include meadowgrass.

Agrostis alba (noun)	meadow grass (noun)
slender European grass of shady places; grown also in northeastern America and temperate Asia	any of various grasses that thrive in the presence of abundant moisture