Caloplaca herbidella, a rare species in the fertile state as seen in this early photograph taken on an ancient ash tree in the New Forest. I apologise for the quality! When fertile, the thallus which is grey, and isidiate as in Caloplaca chlorina, is usually hard to see. It is a little easier when infertile, as in the next photograp, but not often recorded as it is minute. In the infertile state, it seems to be more frequent and is turning up, for instance in  Wales