definately a monti nudi. looks like a full grown adult, which means there is probably many, many more to follow.
I ended up getting one adult in on a frag once, and it ended up laying eggs. Lost The one adult almost took out an entire hand sized red cap by itself, and when the babies hatched they wiped out my superman frag. I had to pull the superman frag and throw the whole thing out, because they were all over it, and I had to cut away almost all of the red cap and was left with one dime and one 1/2 dollar sized frags.
I suggest quarantining all your montis, dipping with lugols or something similar, and cutting away all the affected areas.
Its sucks , but that's how I had to get rid of them.