Car moisture and musty smell problem

[schiddy]

I live in CT Tom. And we're in a cold spell right now. 12 degrees right now. Tried to look at the car this morning as I couldn't yesterday because of work. So freaking cold hard to look for clues. Carpet under the mats seemed dry and didn't see any ice but it's so cold. Didn't see any headline water staining by the sunroof or anywhere else on headliner. I didn't turn on any ventilation in fear of spreading spores and just tried to look for signs of water leakage.

Only clue I could find was some ice on the INSIDE of of the windshield by the defroster vent.




Have the windows and sunroof open now to air the cabin out a little. Is it too cold for this to even be effective? How should I proceed from here? Should I just go ahead and clean the evaporator with klima cleaner?
How to apply einszett Klima-Cleaner

Have to drive to the parents 50minutes away this weekend. Not looking forward to this as this probably won't be solved before then. (Even if I know what it is/what to do, I'll have to order products).