I hope it don't come back. I wish you the best of luck with it staying away.....something tells me it was just because of the addition of the new rock. Was it live rock like cured...or was it dry?...A good example of how things react with there environment is with wood in a home aquarium. When you add wood to a home aquarium your suppose to boil it to help kill bacteria and anything else on the wood. It also helps leach out some of the tannin's in the wood keeping your water from becoming to acidic in the aquarium from said tannin's....though if your using wood your fish type live in an acidic environment anyway. Boiling also help the piece take on water to keep it down....If you don't boil the wood and you put it in an aquarium then you will see this white "fuzz" in patches on the wood by the next day usually. This is a bacterial growth which is completely harmless to the system and some fish will eat it when they find it. However saltwater is a little different in what it grows seeing it can harbor just as much or more life than freshwater (in the way of bacteria and such). If those rocks you added were dry and un-treated before they were put into the aquarium then that could cause growth. It looks like a cyno in your images which would make sense for a un-treated object entering an aquarium....but if the rocks were treated or live then its possible its diatoms. In my personal opinion, because of how the rocks look so new white im going with a cyno bacteria, which is probably on the rocks feeding on other bacteria. If you keep your tank clean im willing to bet once its consumed all the other bacteria off the new rocks it will subside and become un-noticed again, if it doesn't stay away after you scrubbed it......again I hope it stays away and your tank continues to stay beautiful just like it is.
