If you can get the rock out i would say burn the bugger. But you gotta be careful he doesn't just retract back into a hole. If that's not practical they make prepared mixtures like joes juice or Aipastia X which is what I use. But if you have some Kalkwasser around you can mix that to a yougurt like paste and inject them right in the mouth with that. Just turn off your flow if you inject anything into them, give the stuff time to work.
Most people would say just get some peppermint shrimp cause they are supposed to eat them, but I've had zero luck with them eating them for me.
best to nip this guy in the bud before he gets outta control, odds are there are more in your tank you havne't found yet. :-(