Burn it before it starts spreading. I have a large seringe i bought from a vet supply store, im sure you can find in many places, its for horse vaccination. It comes with a large needle as well. Mix kalk into a paste in a little container, and with the seringe drop a few drops on the anemone. The product is very thick and sinks in the water. As it touches the anemone it sticks to it and from there on goodbye little pest.
since you're able to move the rock, take it out and let it sit in a bucket until it comes back out again.
you can use kalk in a syringe, boiling water in a syringe or my personal favorite... a butane torch....:skull::sgrin:
Yeah I wouldn't boil or burn anything. Just chip the piece of the rock where the majano is located and throw it away. You never know what else is on that rock.