I can't really see it very well, but I'm thinking it may be some sort of calcium deposits from the water itself. I don't think it's anything to worry about, and it will probably resolve on it's own as the tank becomes better established. (I'm guessing that it's fairly new, but I may be wrong...)
The tank has been running since last December, but some of the rocks in it were only put in a few weeks ago. Ill try the bumping it idea to see if that could be it...otherwise Im not sure...kinda weird. I might just take everything out this weekend and soak it in bleach water....at least it doesnt seem to be effecting the fish.