algae vs alga

algae and alga both are nouns.

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

As nouns, algae and alga are synonyms defined as:

algae and alga: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

algae (noun)	alga (noun)
primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves	primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves