I've had something like that on my rocks in the old 20L setup...never had it sense. Not sure what caused it honestly. Maybe just a mix of all the right conditions.
Edit: The only thing I've been able to get it off with is H2O2 or Excel and a toothbrush. You can spot dose it in the tank and it'll melt off eventually, but I used to just do it at water change times because I didn't want that crap hanging around in my tank.