My S&B smells like mildew :(

I kept it at 50 degrees while I was away. Apparently that was not enough. There is no water infiltration, roof leakage, etc. I found one tiny patch of mold on the wall behind a knicknack, I assume from condensation, and I am guessing that will turn out to be everywhere air is stagnant.

Is there anything I can do about the smell now? I have been airing out the house for a couple of weeks, doors and windows open all afternoon on sunny days, but it has not helped much. And i am cleaning!