Goldfish belly turning green?

I bought 4 baby goldfish a little over a week ago. The only tank mate with them was a fiddler crab. The other day, the fiddler crab had gotten a hold of one of them and unfortunately that fish didn't make it. But, since then another fish died a day later, then another one. Those two plus the one that is still alive all had their bellies turning green. ): I'm not sure at all what is causing it, if it's a disease or something, and I thought it very weird that the fiddler crab started this whole thing, or perhaps just a coincidence.