green algae vs algae

green algae and algae both are nouns.

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

As nouns, algae is a hypernym of green algae; that is, algae 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
algae: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Other hypernyms of green algae include alga.

green algae (noun)	algae (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