So now that I have time to elaborate, I had one of those crabs in a tank long ago. I thought he was cute too. Then he grew. And grew. And grew. He was about 2" across at the shell, pretty good sized for the 60G tank he was in.
Then I began missing fish. I still didn't want to blame him. Until I saw him brazenly crawl into a firefishe's dart hole, grab it by the face, and begin eating it. It never bothered corals, but after a certain size they can take out fish, even seemingly healthy ones.
I smushed mine with an acrylic rod after the firefish incident.