A contact of mine who's started to breed mussuranas says keepers can't agree on whether it's related to low or high humidity. If anything I'd expect my tank's on the dry side, but I'll start offering a humidity hide around the clock so he has the variation. The dents do seem to come and go with mussies, so I'm trying to not worry too much about it. I was told that as long as they don't become raised bumps/pimple like things (related to too high of a humidity) it's okay.