kelp vs brown algae

kelp and brown algae both are nouns.

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

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

kelp: large brown seaweeds having fluted leathery fronds
brown algae: algae having the chlorophyll masked by brown and yellow pigments

kelp (noun)	brown algae (noun)
large brown seaweeds having fluted leathery fronds	algae having the chlorophyll masked by brown and yellow pigments