Confidence: It's likely to be this, but I can't be certain.
Notes: It is very unwise to identify a Porpidia without microscopic examination, let alone from a photograph, but I feel I should suggest an alternative. It is not, I am fairly sure, P. macrocarpa - the thallus is too thick and well developed (usually thin or immersed into the stone surface in P. macrocarpa) and the apothecia are more or less set in the thallus ("innate") (sessile but contracted under the apothecium in P. macrocarpa). The white thallus and innate apothecia which look like they are pruinose when young strongly suggest P. cinereoatra, which can also have clustered apothecia as seen here. But Porpidia is a large and very difficult genus.