red algae vs alga

red algae and alga both are nouns.

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

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

red algae: marine algae in which the chlorophyll is masked by a red or purplish pigment; source of agar and carrageenan
alga: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Other hypernyms of red algae include algae.

red algae (noun)	alga (noun)
marine algae in which the chlorophyll is masked by a red or purplish pigment; source of agar and carrageenan	primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves