Man vs Pin says it can’t be done with a moss/buttermilk slurry.He tried it in LA on a dry wall and used tap water to mist it. He also says the graphitti idea came from Anna Garforth who used sheets and cut out her letters.
http://www.annagarforth.co.uk/work.html
Instructions for propagating moss:
True evidence of moss propagation with a slurry like idea
More encouraging – I will try it. But he also advises against beer or buttermilk saying it will lead to mold.
Traditional moss transplant:
Project Moss 