I've not been able to get emerald Crabs to eat large valonia, however, they will eat the smaller ones that grow after a large one has burst. When I have valonia issues I purchase the crabs, pop the larger (bigger than a pea) valonia, and crabs take care of the rest. The crabs live long enough, a couple of years, for all of the cells of the popped valonia to settle and grow.
Hope this helps,
-Greg