In general once a haddoni finds a spot that makes it happy, it stays put. They don't move around much, unlike BTA's. If you choose to keep any Haddoni, you must accept the fact that any fish in the system are possible meals for the nem.
One other potential issue is that they can get quite large. I have 4 in my system. One of them is so large, I have had to move most corals from about 1/3 of the tank to give it the room it requires. I have moved several corals since this picture was taken. The monti cap, galaxia, frog spawn, and leptastrea have all been moved further away from the purple haddoni for their safety.
20200406_170654 by
Lyle Southard, on Flickr