i have one for over 10 years now. I dont remember the last time it move. Just make sure there are no corals surrounding. Cant say the same for my GBT too many splits and they move all around. I have to get rid of them
As mentioned they can crash a tank. I have experienced both a nem that literally seemed to melt overnight to the unavoidable "egg drop soup", when the nem checks out the gyre. In my experience if the nem gets chopped up its not nearly as bad as when one melts down. There are plenty of warning signs before that happens usually. They do move around and they can sting things but I haven't noticed that as being so much a problem as them simply deciding to move to a piece of prime real-estate that you wanted to put some coral on.