I actually attempted to do my own moss art two years ago for my biology class in high school, but unfortunately it didn't turn out very well. Here's a moss paint recipe that looks promising. Perhaps someone else would like to give it a go!
Moss graffiti, also called eco-graffiti or green graffiti, replaces spray paint, paint-markers or other such toxic chemicals and paints with a paintbrush and a moss "paint" that can grow on its own. As people become more eco-friendly and environmentally aware, the idea of making living, breathing graffiti has become a more green and creative outlet for graffiti artists. It can also be considered another form of guerrilla gardening.