Peppermint shrimp do not eat Majano sp. in my research.
Instructions (from another site but have seen it work in a friends tank)
First - Get the aquarium ready by turning off the power heads and pumps. It is much easier to apply the paste in an aquarium with no current. You also reduce the risk of harming anything other than the majanos.
Second - Mix the Calcium Hydroxide (Kalk, or pickling lime which is cheaper) powder with just enough water to make a thick paste like the consistency of toothpaste but just a little thinner.
Third – Apply the paste over the whole majano using your applicator.
Fourth – Turn the power-heads and pumps back on after approximately 30 minutes. Watch to make sure that none of the paste lands on any corals and if it does blow it off quickly so they are not harmed.
The other option if they are on only one rock if you can get it out of the tank. Sprinkle some regular table salt on the majano. they will release and fall off.