Yeah I have not read before that peppermint shrimp eat majano but who knows I guess.
Can you take the rock with it out of the tank? The best way to get rid of these critters is out of the tank, regular table salt on them and they fall off the rock. From there you can do what you wish, Tapas, hors d' oeuvres, dried out for trail mix etc. etc. (J/K on the food part I hope you know)