Bright Cobblestone Lichen (Acarospora socialis) is very common in southwestern United States. It is the most common yellow member of its genus in the southwest, and one of the most common lichens of any kind in the deserts of Arizona and southern California. It is uncommon in Minnesota, where it has been recorded only in Cottonwood County.

Bright Cobblestone Lichen is up to 4″ wide and may be crusty, appearing sprayed on like paint; leaf-like, with thin, flat, lobes; or cracked, appearing somewhat like cracked paint. The upper surface is usually bright yellow or greenish-yellow when fresh but may be bleached white with age.

http://www.minnesotaseasons.com/Fungi/Bright_Cobblestone_Lichen.html