Many times people jump to conclusions about mold killing their ants. The ants do a really good job at keeping mold under control, and when they die, many times they are covered in mold the next day, leading people to believe that the mold killed their ants. I'm sure it happens, but probably less often than people think. I think if it does kill the ants, it's probably the CO2 the mold produces.
Aspergillus flavus, a pathogenic species characterized by a yellow, hairy growth on organic material, such as dead insects in the ant nest, does actually produce a number of potent mycotoxins that may be deadly to ants in the proximity of the mold outbreak.
Here is a small outbreak currently ongoing with one of my colonies. I'm keeping an eye on things to see if the ants can manage it, or if it needs my intervention. For now, I'm just discontinuing watering and limiting feeding so there's not an excess or moisture or uneaten food in the nest, both things that the mold needs to grow further.
I hate that stuff. Always on dead things.