Top down and front shot.
It really seems to like very white surfaces, its starting on a upside down clam shell in the one picture.
It could be just large white pieces of sand or crushed clamshells that it is growing on?. Grab a piece , you should not be able to rub or wipe it off at all
without scratching it off
What type of lighting are you running?
I can remember the pieces of LR that had the green coralline on it, I had them under PC lighting for over a 6 months and any sign of green,pink or red coralline was had faded untill I moved the rocks into my 110g with the MH lights.
Peace-out