Caloplaca chlorina. A common species of a variety calcareous habitats. It is relatively easy when fertile, which it often isn't. In this photo it has well developed thallus which is blue grey consisting of squamules edged in isidia. The thallus is often darker, and less well developed. It used to be called Caloplaca isidiigera. This photograph taken in  Lingby Churchyard, Nottinghamshire