Im lovin' lichen

[Lilith7]

The theory is you grab a clump of  flowering moss, bung it in a blender with some yoghurt, whizz it up and paint that onto a damp dirted up surface, and keep in the shade for a few months.

Try it and report back...

Bugger that - my food processer's temperamental enough now, I hate to think what it might do if I put moss & yogurt in it! 
Probably do an exorcism all over the kitchen, spewing moss & yogurt merrily everywhere.