green algae vs alga

green algae and alga both are nouns.

Word	Noun	Adjective	Verb	Adverb
green algae	Yes	No	No	No
alga	Yes	No	No	No

As nouns, alga is a hypernym of green algae; that is, alga is a word with a broader meaning than green algae:

green algae: algae that are clear green in color; often growing on wet ricks or damp wood or the surface of stagnant water
alga: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Other hypernyms of green algae include algae.

green algae (noun)	alga (noun)
algae that are clear green in color; often growing on wet ricks or damp wood or the surface of stagnant water	primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves