seaweed vs algae

seaweed and algae both are nouns.

Word	Noun	Adjective	Verb	Adverb
seaweed	Yes	No	No	No
algae	Yes	No	No	No

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

seaweed: plant growing in the sea, especially marine algae
algae: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Other hypernyms of seaweed include alga.

seaweed (noun)	algae (noun)
plant growing in the sea, especially marine algae	primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves