yellow-green algae vs alga

yellow-green algae and alga both are nouns.

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

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

yellow-green algae: any alga of the division Chrysophyta with its chlorophyll masked by yellow pigment
alga: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Other hypernyms of yellow-green algae include algae.

yellow-green algae (noun)	alga (noun)
any alga of the division Chrysophyta with its chlorophyll masked by yellow pigment	primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves