I don't know the name of the coral on right, maybe a Pavona. My Monti is being overtaken by the Pavona and want to remove or kill it. Removing the rock is not an option. Any help appreciated.
You could also put aiptasia x on it, it will kill the areas you put it on. If you go the epoxy route your gonna need quite a bit. You could also just physically remove it, chisel away at it and scrape it down to nothing.
Most LFS sell the tubes of putty you mix up. That works well on nuisance corals IME. I've covered zoas, palys and encrusting SPS with it many times to give the neighbor coral a chance to grow.
The JB WaterWeld epoxy is pretty popular. This can be found at most hardware stores for like $7. Just knead up a small ball, flatten it out a bit and then cover what needs to be covered. Piece of cake. It's basically like putting chewed up bubble gum over something.