Well first of all, my problem has been solved with the help of a friend. Turns out it was spider mites, which I know nothing about, and I mistook it for mold.
But the reason I dont think I can add organisms is because its an open terrarium. It has no lid. Plus the soil is only about nine inches from the top. It's only a cleaned out 10 gallon fish tank.
I know I could make a lid with acrylic, but some of the plants I picked are too large and several are over the top.
Springtails jump, and could easily jump out I suppose? I'm not sure, I know they jump, but I've never used them yet. This is my first terrarium.