rockweed vs brown algae

rockweed and brown algae both are nouns.

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

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

rockweed: coarse brown seaweed growing on rocks exposed at low tide
brown algae: algae having the chlorophyll masked by brown and yellow pigments

rockweed (noun)	brown algae (noun)
coarse brown seaweed growing on rocks exposed at low tide	algae having the chlorophyll masked by brown and yellow pigments