First off....this happens to everyone. It likely just didn't survive the acclimation stress.
Acclimating corals is far from an exact science, but you can almost guarantee that the tank parameters that it came from is different than yours. Alk,ph and temp are probably the biggest thing to be mindful of when acclimating a coral. But even with the most care, this is stressful to the coral, and sometimes even when you do it right, it doesn't live.
But I would leave it be, montiporas can recover from the smallest spec of living tissue.