nudibranch. Technically I believe related to snail, but this guy is usually not welcome. Unless it was specifically added (aiptasia eating), get it out.
So sorry. These guys can be pests, but they can be treated. Get it out. If you can remove your monti, look for more and eggs. Scrub them off or use a water pick to shoot off. Then dip in a coral rx, iodine, similar. May have to use something stronger to get them all. Good luck. Hopefully you can save the monti.