You could do a 50/50 mixture of coconut fiber and organic top soil (additive-free) that doesn't contain mulch. To help retain humidity and resistant to mold/fungal growth. I don't know any other factors really promoting your mold growth other than what has already been said along with: Spot cleaning (Dead or decaying matter, poop, etc.). Disinfect anything that is in the terrarium that could potentially be harboring spores for fungal growth. In your case you'll just have to change it every 3-4 weeks and if mold appears remove the top layer until cleaning time. That is all I can suggest for now.