I wouldnt worry. I had an Emerald die and I had a YT Damsel die and everything in my tank lived. I have a 55 gallon with 110 lbs of live rock in it. Plus, I have a 400GPH filter, 2 450 GPH power heads. I dont know what you have, but my guess is that one tiny little crab wont foul your water up...
Go buy another, throw him in there and he'll eat the other one in no time flat