Placopsis cribellans

Placopsis cribellans is a species of crustose lichen in the family Trapeliaceae. It forms small, neat rosettes or irregular patches on rock, with a surface that appears pale green when wet and shifts to grey-white or fawnish when dry. The thallus bears tiny spherical isidia that break off easily, leaving characteristic pits, and produces reddish-brown disc-shaped fruiting bodies. The species has a wide distribution, with records from East Asia, the Pacific islands, Australia, and both North and South America....read more on Wikipedia.