If it's nori, cerianthus is right, it will likely fall apart and end up in your filter (I had that happen twice, then I learned my lesson

). The stuff is kind of strange. Dry, the stuff tears easily. Moist, wrapped around sushi rice, it's fairly tough stuff. You can bite through it, but it doesn't rip as easily. Wet, it falls apart.
Only feed enough nori to be immediately eaten.
Other types of sea vegetable can last longer. Certain types of kelp are cut in whole strips (rather than a pulp that's pressed together like nori). However, these can be tougher to eat, and soft-mouthed fish may have problems with them.