protoctist vs alga

protoctist and alga both are nouns.

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

As nouns, alga is a hyponym of protoctist; that is, alga is a word with a more specific, narrower meaning than protoctist:

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

Other hyponyms of protoctist include protozoan, protozoon, algae.

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