Rhizocarpon

Rhizocarpon is a genus of crustose, saxicolous (or sometimes lichenicolous), lecideoid lichens in the family Rhizocarpaceae. The genus is common in arctic-alpine environments, but also occurs throughout temperate, subtropical, and even tropical regions. They are commonly known as map lichens because of the prothallus forming border-like bands between colonies in some species, like the common map lichen (Rhizocarpon geographicum)....read more on Wikipedia.