I have kept a small green one - have absolutley no idea what it really was. Most LFS will just get them in as 'mantis shrimp', irregardless of size or color it seems. I got mine around 2", threw it in my sump (I have live rock in it) and saw it 4 times. Pieces of shrimp and other meats dissappeared for about a month, but eventually I started noticing that the fireworms were the only consumers..never saw it again, and I can only assume it is gone..
I did see another 2" inch 'blue' mantis shrimp at the LFS the other day, and in the same tank was a 3 inch yellow-head jawfish, and a few hermits..The mantis was out and about, didn't attack the other inhabitants at all. I personally wouldn't trust them with anything though.