Fucus serratus vs brown algae

Fucus serratus and brown algae both are nouns.

Word	Noun	Adjective	Verb	Adverb
Fucus serratus	Yes	No	No	No
brown algae	Yes	No	No	No

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

Fucus serratus: brown algae seaweed with serrated edges
brown algae: algae having the chlorophyll masked by brown and yellow pigments

Fucus serratus (noun)	brown algae (noun)
brown algae seaweed with serrated edges	algae having the chlorophyll masked by brown and yellow pigments