Could potentially work, you’d need something that grips the moss paint enough to hold the design, but not more than the wall surface. That’d be tricky.

I knew a couple street artists who pre-printed all their work, they used wheat paste to slather it onto back alley walls and painted over top. If you used a cheap, recycled paper that decomposes just a little slower than the moss roots in, that could work to just glue it in place.

Stencils are my personal go-to.