Blender the moss with some juicy nutrient rich tank water, then paint it all over the rocks.
Mist once a day.
I'm not sure if this form of "graffiti'' has hit your city yet, but with Vancouver being so full of art student/hipsters, you can randomly find words and pictures on the sides of buildings from people who do this.
They blend the moss with water and nutrients (probably old tank water) and add a little bit of buttermilk (not exactly sure what that does) and then they use a brush and paint words like "Love" and "Beautiful" or whatever on the sides of buildings. Secretly at night they mist them and then randomly like a couple weeks later there is a word on the side of a building ''painted'' in moss...