They usually start at the edges because they hide under the coral during the day. Especially caps, they will turn the edges white and work inward. something like digitata they will start at the bottom/base and go up.
That said it might not have been them. maybe a combo of things too.
Also there are so many nudies out there it might not be monti eating nudies but I suspect they are. Only way to tell is check you corals at night, they also prefer certain ones. They also seem to lay in the part that is white at the edge of the tissue making them hard to see too.