Peltula euploca

Peltula euploca, the powdery rock-olive, is a species of lichen-forming fungus in the family Peltulaceae. First described in 1799 by the Swedish lichenologist Erik Acharius, the species was later transferred through several genera before molecular studies confirmed its placement in Peltula, the sole genus in Peltulaceae. Its overall appearance is that of a powdery-edged, olive-green rosette closely pressed to the substrate. In dry conditions it often becomes dull and brownish but can quickly rehydrate and turn green when moistened. Each thallus is attached to the rock by a single central point (an umbilicus) and spreads outward as a small shield-shaped lobe, typically only a few millimetres across but sometimes reaching about 12 mm in diameter. The lichen rarely produces fruiting bodies; instead, it reproduces mainly through powdery soredia that form along the margins and give the thallus its characteristic dusty border. It grows on exposed siliceous rocks in arid and semi-arid regions......read more on Wikipedia.