algae vs green algae

algae and green algae both are nouns.

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

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

algae: 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

algae (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