That's a Cymo andreossyi not a commensal crab. I would ditch him because it feeds off polyps but they aren't technically the worst thing to have. Here is some info.
I have 2 of them just like him living in one of my acro colonies, and if they are eating polyps, I cant notice and its not bothering the coral one bit. Acro's color is great, polyp extention is great and it grows fast. So I say keep him. Even if he eats a polyp here and there, it will not be anything you will ever notice. The 2 I have stay in the acro all the time. I have had them for about a year.
Heres the part that says basically that they may eat a polyp here and there, but they still do the acro a service and help keep it healthy.
Cymo andreossyi : Very common and is considered a coral symbiont, with the caveat that such symbiosis are a trade off between the coral and the crab, the coral gains protection and house cleaning services while giving up a few polyps, mucus and captured/settled food particles in exhange for those services. A healthy coral should have no problem repairing or replacing lost polyps, but again, in an aquarium environment, an eye should be kept on the coral for excessive damage being done due to the coral being unable to recover as fast or faster than the damage being done by the crab. In short, its a judgement call that you will have to make.
I had one in a wild Millie I bought, I saved him from the dip and he turned on me and destroyed the Millie... I would flush him personally... Blue or red eyes are devils hah...