yes, you can
sushi is just the seasoned rice... you could take a small hand full and put a veggie or some tofu on top, you will have a vegan sushi.
by the way, the nori, which is the green "sea weed" is not actually seaweed, its a green algae.
if you want the nori-like material to wrap or tie your sushi, you can basically just use a dark leafy green as a replacement