sea lettuce vs green algae

sea lettuce and green algae both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea lettuce	Yes	No	No	No
green algae	Yes	No	No	No

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

sea lettuce: seaweed with edible translucent crinkly green fronds
green algae: algae that are clear green in color; often growing on wet ricks or damp wood or the surface of stagnant water

Other hypernyms of sea lettuce include chlorophyte.

sea lettuce (noun)	green algae (noun)
seaweed with edible translucent crinkly green fronds	algae that are clear green in color; often growing on wet ricks or damp wood or the surface of stagnant water