Either I'm reading you wrong, or you're misunderstanding what I was saying.
Montipora eating nudibranchs are a type of "sea slug" that can hitchhike on corals and will consume the corals in question. They are quite small, and sometimes hard to see as they contrast very well against the coral, easiest way to see them IMO is to try too look at the edge of the coral with a flash light on it and you might see something moving around on it, big ones being smaller than a small grain of rice. Now if your white spots are not growing it could be a good sign, as the nudibranchs will eat the whole thing leaving you with a white skeleton, but it will take time.
As to distance of placement of the corals? Don't think it really matters much, most montipora are not that aggressive towards their own species (at least unless they are absolutely touching) and then it's usually a "who can grow faster" race. So I wouldn't worry too much about placement.
