Funny you mention this, I had the same problem this year and I never have had this problem before. The couch, and chairs seemed to have been the worst, but it covered other areas in the cabin also.
I ran my A/C back in July for about 4 days, and left and closed everything up. I am guessing that was the problem along with it being a exceptionally wet summer. I left a pair of wet saddles in one room and they were covered in mold. Now most of it wasn't growing by the time I found out, but it was a process of cleaning everything and not worry about getting sick. It put like a thin film of mold on everything. I used disinfectant and they say that kills the growth, so we will see. I used a vacuum on all the cloth material, and I never got sick so it must have not been the kind that makes you sick.
I think what happened on how I got it in the cabin was coming in from being in the river all day and wearing my trunks that had dried out but still sitting on the couch and chairs. Which most likely left some of the moss, spores etc from the river. Then closing up the cabin tight.
I hadn't been down to our camp in two months when I went Friday afternoon. Everything inside was damp feeling. I had the air set to come on at 85 and the heat at 55 so I guess it was to much difference not too mention being sealed tight for two months. Just a few white spots here and there mainly at the tub and shower. I turned the ceiling fans on in every room and set the air on 70. It had pretty much dried out by Sunday afternoon. I think the answer is use it more, at least that is what I am telling my wife.