Crabs are said to be opportunistic eaters & SOME OF US (not no where near all) don't want to chance having a crab in our tanks. I tried a Coral Banded Shrimp (CBS) and that didn't work out for me (took him back to the LFS) because he would start moving around after the lights went out with his pincher claws extended looking for food & killed my peppermint shrimp. It was NOT easy to catch him out of my tank so NO MORE CLAWS are going in my tank. I'll fight the Bubble Algae manually as long as I can see I'm staying on top of it.
Letting your rock sit out to dry will also kill the beneficial bacteria on & in it.
It's easy to PANIC over pesky algae growth. I've done it SEVERAL times only to realize that I shouldn't have. This Green Bubble Algae (GBA) won't defeat you IF you stay on top of manual removal. It's not going to kill anything in your tank at the slow rate they appear. That's my opinion.
Make it a game of seek & destroy....it's even kinda fun if you look at it as a game