Yea, I've had emerald crabs before. They are pretty cool, but will eat just about anything they wish. At first when they were very small they never messed with coral. Once they got a little bigger I noticed one nipping at my coral pretty good so they had to go. Wasn't worth the risk to me. So far only thing I know that it reef safe and with eliminate bubble algae are Tangs, rabbitfish, foxface and such. And they need a big tank. Some angels will, but they can nip at coral as well. Ive though about giving a pygmy or flameback a try, but just not feeling risky enough. Hopefully Ill get this coralline algae growing back good like it once was (now that my harlequin is devouring the asterinas that ate it all) and it will outcompete the bubble algae. Been a little while since I tested my phosphate and changed GFO too. I probably need to go get on that. I have heard some success stories with the emerald crabs though. Keep them well fed and they probably wont go after corals. But then again who is to say the will go after the bubble algae when well fed too. Worth a shot but keep an eye on em.