seaweed vs alga

seaweed and alga both are nouns.

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

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

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

Other hypernyms of seaweed include algae.

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