Walls are sticky

[Bienes Raices]

My offer was just accepted on an REO. There is visible mold in one closet (photo below), but also the walls in some of the rooms seem to have a sticky feeling. I don't mean that there's actual goop or liquid on the walls, more that the walls have kind of a tacky feeling to the touch (kind of hard to explain without actually touching it). Does this usually mean that there is mold or wetness present throughout house? What causes sticky wall? If it matters, the AC was removed so it's warm in the house.