To add to what others have said:
I would leave the cover off of the terrarium that you have made.
High humidity and stale air aren't good mixes for a VFT. They enjoy a more open condition.
I believe that the mold you have is from the conditions that plant was in ( and not the soil- but I will address that later ). The high humidity and no air flow makes mold. Also, some have thought that certain water will too.
Distilled, rain, or water from a reverse osmosis filter is the best things for the plant. Tap, drinking, spring, mineral...all contain salts ( sometimes not listed ), minerals and other chemicals that are harmful for the plant. The plant will not die right away, but over time, the build up of these things will harm the plant and will eventually kill it.
Soil:
You have the right mix now. When planting VFT's only use Spahgnum Peat Moss, Perlite or Sand ( that has been washed ). There are other things that people use, but for those that are just starting, they are the easiest things to get.
Since you have the plant in a tank, you want to also make sure that you aren't adding a lot of water to the soil. It is very different than having the plant in a saucer of water.
The best way to water is stick your finger in the soil. Feel how mosit it is. When it starts to dry slightly, add more water, But not a lot. Just keep it moist. You have to think about the water that is at the bottom of the bowl.
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My only question that I have is about your lighting
What do you have setup for the plant? That will also help in the prevention of mold.
I wish you all the luck with your plant. They aren't hard to keep at all. There is just so much bad information, or in some cases, not enough.
Please keep us posted on your baby!!