I have a 14 gal biocube with this problem too. I turned the lights out for 72 hrs to help rid a diatom bloom. When the lights came on their was spots of coraline everywhere. I'm still waiting to see if it sticks around or grows (it's been a few days.)

Note: I seem to get better results on my 58 gal. when I rotate a 2part with my kalk drip.