Seaweed generally refers to the leafy/mossy stuff that is attached to rocks and sways in the tide. As a term, it sometimes refers to actual marine plant-life, and sometimes to algae-cousins that are physically similar to plants but evolutionarily different (such circumstances are called convergent evolution) Algae generally refers to the colored soupy stuff that is gunk/mold-esque, like what you might find in a swamp or a really gross pond. Most of it is single-celled goop. Call it “algae” with no capital letter. Algae, capital A, is also a much larger grouping of “simple” forms of life, of which both “seaweed” and “algae” are members, as are Lichens and Coral. So, Certain kinds of seaweed are Algae, some seaweed is not Algae at all. Seaweed is not “algae” (the unicellular stuff). Also, there are a lot of kinds of algae that are not seaweed, but are still Algae.