Moss for a Vivarium

[Shadowfax]

I had read that you can make Moss paint so I took plastic screen and painted the screen on the back of my tank along with bark and branches. It has begun to grow mold. The left over a batch of moss paint I put out side on concrete and shaded area, and it has grown mold. Is this normal or should I start again? Thinking, buttermilk and beer why wouldn't it not mold.
any advice on this? Thank you much.