Moss skull-and-crossbones by Ladybird at the Instructables.
I came across a step-by-step guide to creating moss graffiti at the Instructables that is different than the other two methods I’ve seen before: painting a moss slurry onto a hard surface (which I’ve tried, unsuccessfully), and removing living chunks of moss and placing them in the desired pattern. Rather, it’s kind of a combination of the two.
Basically, Ladybird recommends using the moss slurry mixture to start your graffiti on a layer compost in a seed tray. You then keep it well misted until established, then transfer to its final display space. Makes sense, and probably give your graffiti a fighting chance.