My crabs actually like those green cubes called Formula 2 by ocean nutrition. They dont fall apart and the fish like them too. I have also used the seaweed sheets, but they are messy and unless eaten quick they start flowing around in the tank and clog your stuff depending on how much current you have. My clowns, damsels, and fox face also like the cubes but wont touch the sheets
I read somewhere that the emerald crabs can start nibbling on other stuff if not well fed.