Zooanthid polyps like that I just let encrust and grow over a piece of rock adjacent to them. If you have any small bits of rubble or LR, or even old Snail shells (if big enough), work just fine. Pile them up next to the colony, as it grows it will grow over the small pieces. When covered, simply rip them off the mother colony, and you've got yourself a frag. I wouldn't worry about poisoning your tank, I regularly mash them up and make a mess of my Zooanthids when rearranging things. They let off some slime and don't seem very happy overall, but in a matter of days they're back to their usual self. If you just want to split the colony you have, I'd remove the rock from the aquarium, get a hammer and screw driver... You know what to do. Don't worry about the corals, they're tough buggers, and will make it through just fine.
- Mac