I've been having some problems here lately. Been having mold problems.
I think a lot of the problem is the ants can't adjust the air flow through the nest, most of the nests I have only have one opening so there is no air flow.
I read they will add tunnels and block tunnels to keep the nest perfect. The out worlds have good ventilation but its not enough to keep the nest from molding. I haven't been over watering, there are no food items in the nest. But what's molding is their bathroom area. I'd add some of the book lice from my infected colony but I don't what them to get out of control.
We burn wood and I use a dehumidifier so the inside of the house is dry.