I don't buy the "dirty water" theory since in the ocean, the water is constantly changed. Sometimes some corals do better than others and we may never know why.
I had xenia growing out of control in a tank a few years back. It was a plague. I was reduced to selling giant rocks for the weight of the rock just to get rid of some of it, until one day, it stopped growing. It never grew back and I wasn't even able to re-introduce it into the system again. It would die as soon as I would add it.
Nothing changed. Same system, same water change schedule, same everything.