sargassum vs brown algae

sargassum and brown algae both are nouns.

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

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

sargassum: brown algae with rounded bladders forming dense floating masses in tropical Atlantic waters as in the Sargasso Sea
brown algae: algae having the chlorophyll masked by brown and yellow pigments

sargassum (noun)	brown algae (noun)
brown algae with rounded bladders forming dense floating masses in tropical Atlantic waters as in the Sargasso Sea	algae having the chlorophyll masked by brown and yellow pigments