Moss painting is the process of creating an image on a surface, usually of stone or wood, where the "painter" applies a mixture of moss and some sort of nutrient mix and grows it into the intended depiction. Although sometimes used in "eco-graffiti" it is an astonishingly poor medium for any kind of actual vandalism, as the moss will likely require multiple coats of mixture and the frequent application of moisture to ensure healthy growth unless perfect conditions prevail, in which case your graffiti is in a shaded, wet place, likely out of sight of anyone you would wish to display the work to.
I have but one experience with moss painting. In high school, during the summer of what I think was our sophomore year, I was friends with an intelligent, enthusiastic young woman who was still, before being hardened somewhat by the world, prepared to throw herself headfirst into whatever interesting projects struck her fancy. She acquired some cheap beer from one of her older cousins specifically for this purpose (cheap beer being a recommended base for the nutrient mix when mixed with some sugar at the time) and wasted little time roping myself and one or two mutual friends into an attempt to paint the town green.
We gathered the moss from other people's lawns over the course of an hours walk on a warm and pleasant summer day and threw the chunks of greenery and dirt into plastic bags. There was not much thought given to what kinds of moss we were gathering or its' living preferences and there may have been several dozen different species of moss in our bags or maybe just one in several different stages of the bryophyte life cycle... Obviously, none of us had biology degrees at that point and I never would, despite a half-assed attempt a few years later. Regardless, the "nature hike" portion of moss painting comes across from my memory as a highly recommended experience although I do not condone the commission of wanton vandalism upon other people's lawns; go to the park.
We bought the bags to her house and, without thinking much of it, separated out only the larger rocks and clumps of dirt (and there was some debate over how much dirt should be allowed) before throwing the remaining wads of moss into her family's blender along with some processed sugar and as much PBR as we could fill the glass pitcher with. The blender made a terrifying rattling noise as it reduced the beer, organic matter, and sugar into a pleasantly alcoholic and earthy smelling yellow-brown solution, although mixture might be more accurate as much of the dirt and remaining small pebbles rapidly settled on the bottom of the pitcher much to the chagrin of those who had protested for the inclusion of more "nutrients" earlier (mostly myself). In the end we poured it all into a bucket, stole a chip brush from somewhere, and went back outside to do what we were going to do.
My memory is imperfect regarding what exactly we painted that day. We plastered the local telephone poles and underpass with at least one obscene word and some geometric shapes... I personally remember opting for demonic grins and at least one "V" symbol from V for Vendetta (I was a strident fan of Alan Moore's in those days and clearly imbued with a deep understanding of anarchist thought) and we all worked cooperatively on an enormous narwhal on a wall beside our host's garage. At the end of the day there was still a great deal of "moss" leftover... I have no idea what we did with it other than I am reasonably certain we did not dump it into the street as one of my final memories of that day is playing frisbee outside of her house and smelling the cleaner cooler evening air as the sun set... I would have almost certainly remembered it otherwise if we had let it all run down the gutter. She told us she misted the narwhal regularly but in truth we all knew that none of it ever grew.
Here's a recipe for moss painting if you ever want to try it with your good friends:
-A couple of clumps of moss
-A cup and a half of buttermilk. Or 12 ounces of yogurt. Or twelve ounces of beer if you're not a wuss.
-Optionally, if you're serious about moss painting, add some kind of water retention beads or something. The internet is filled with horror stories about people who got their moss started only to have it dry out.
Basically, chuck it all in a blender while being as careful as possible to remove as many of the rocks and pebbles as you can. Dirt is optional and will settle out anyway. Paint your surface horizontally if possible and consider using a stencil. If your canvas is mobile, put it in a shaded area where it won't receive the full brunt of the sun. If not, try to shield it and ensure that you mist it everyday.
Most importantly, have fun! Chances are good that you will fail anyway, so just try to enjoy the company of those you're failing with.