Terrarium? Let it breathe. Air flow is bad for mold. Ive never kept a sealed terrarium or vivarium, but I also kept animals in mine that required fresh air flow. What kind of wood are you using? Coco tends to be heavily innoculated with trichoderma a beneficial fungi, and holds more air in the mix than soil while also retaining a decent amount of water. What does this mold look like? Beneficial fungi doesnt always look pretty either. If intent to treat it there is a product called Actinovate that may help, and other products still to try as long as these terrariums hold plants only, if reptiles, or amphibians are present, especially fish then it limits the spectrum of product options.