Mold in Cold Frames?

So after a cold winter here in 7b finally opening up my cold frames the last few days and just noticed mold inside them. Anyone else have this issue and how concerned should I be? 

I know ventilation is important and have propped them open above 45 F and barely watered all winter, so has not been damp in there (I watered today for the first time so that's why it looks wet in the pics).

Just built them last Oct so surprised to be seeing this.

Thanks!