Emerald crabs, just like every other creature pretty much...are about 50/50. Sometimes they work, sometimes not. Even if you find one that will eat it, as soon as they pop open the bubble, the algae will release spores...and reproduce. Your best bed is manual removal. If possible, I'd remove a bit of rock where the algae is attached as well. This algae is one you really want to nip in the bud immediately. I've never had an outbreak of it, so I can't chime in too much as for widespread removal.