As a city gardener, I like to see quirky stuff people are doing in public spaces. The photo is from an installation at a Philadelphia subway station, using moss. I want to find a place to try moss graffiti. Most of my house's brick surface is too sun-baked.
You blend up chunks of moss with yogurt and sugar. Then you paint it on the surface you want to decorate, like a brick or stone wall. You can spell things, stencil, make patterns. Here is a how-to from the Instructables site. More examples here, here, and here (click moss graffiti).
Eventually it makes a real moss colony and the pattern is lost when it grows, but I still think it's cool. I may propose a public project at our local community art center, the GoggleWorks, which has plenty of brick walls. I think it would be a fun project for kids.