alga vs protoctist

alga and protoctist both are nouns.

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

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

alga: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves
protoctist: any of the unicellular protists

alga (noun)	protoctist (noun)
primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves	any of the unicellular protists