alga vs green algae

alga and green algae both are nouns.

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

As nouns, green algae is a hyponym of alga; that is, green algae is a word with a more specific, narrower meaning than alga:

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

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