I found this recipe for Moss Graffiti on Instructables.
In a blender combine:
1 1/2 cups buttermilk
A few handfuls of moss
1 teaspoon of sugar
Blend until smooth enough to paint (mixture will be thick).
Paint on a porous wall (such as brick or concrete) and mist with water daily until it grows.
Paint freehand or use a stencil and tape if you want it precise.
I am so excited to try this out!