Montipora eating Nudibranchs are very small, like grains of sand, and typically white. The Nudibranch lay their eggs on the underside of the Montipora, which you can see when you turn the coral over. They will eat away at the Montipora and you will see patches of white, where they have eaten.
You can get get rid of them by using wrasses and certain Butterfly fish. As for chemical, an iodine dip and concentrated Lugols does seem to work but not on the eggs, which you need to remove the coral and scrape them off the underside of the coral.