Bubble algae=emerald crab?

[MikeP]

I'm noticing a small amount of bubble algae on some of the LR in my tank, a LFS had some small emerald crabs. Should I go get one? How quickly can this algae spread. I'm afraid if the crab eats all of it he will start to dine on corals or zoos. Would he do that or would he just scavange for left over food?

Thanks for any info