I have a purple lobster too.
I bought him on SWF.com when I first put up my tank about a year ago or so.
In the ad it says they're reef safe so with doing little to no research I bought him.
He is in my reef tank and hasn't done anything to anyone.
After I bought him several fish died but after investigating and a strange tap..tap...tap I heard every night and that all the fish died in the exact same area I found myself a little MANTIS SHRIMP!!!
Just to see if was still alive I have stayed up til 12:21am EST and shined a flash light into the tank I saw those two orange eyes for about a minute.
He has moved for the third time and this is the first time that he isn't visible from the front of the tank.
I hope this has helped you out a little
-Philip O.