brown algae vs alga

brown algae and alga both are nouns.

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

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

brown algae: algae having the chlorophyll masked by brown and yellow pigments
alga: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Other hypernyms of brown algae include algae.

brown algae (noun)	alga (noun)
algae having the chlorophyll masked by brown and yellow pigments	primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves