If it looks like montipora polyps, but seems to have grown out of nowhere there is a good chance it's actually pocillipora. Pocillipora often multiplies via "polyp bail out" where live polyps sort of eject from the original colony and then settle on a substrate and grow into a new colony. You easily could have gotten a frag or rock that had the pocillipora settled on it but not noticable until it grew some. (of course montipora is also possible, but this sounds a lot like pocillipora. They look super similar when it's just a cluster of encrusting polyps.)