Confidence: I'm as sure as I can be.
Notes: Again I am putting in a vague label to counteract a name it definitely isn't. Rhizocarpon geographicum is conspicuously bright yellow, with black apothecia sunk down between the thallus elements (areoles). Here the apothecia are not at all sunk into the thallus surface and I don't think it is a Rhizocarpon. More likely it is a Porpidia or a Lecidea, just possibly L. lactea with a thallus less cracked than usual. But with the huge number of species that are in Lecidea, Porpidia and allied genera, chemical tests, microscope work and THEN speculate on identity.