This is one of several similar species of crustose lichen and as I am no expert I am going to have to play the 'percentage game' and stick my neck out and say that this is Collema cristatum. I am fairly certain it is a collema species and that means it is one of four possible species. All are widespread and common so for the non-expert this is a challenge!
Here is my logic! C tenax occurs on earth, sand and brick mortar not on rocks so three left. C auriculatum has lobes up to 3mm across, this species is muuch finer so two left. C crispum is the slighly more 'chunky' of the two remaining (no point in using the technical term here because lichen have a language of their own!) leaving Collema cristatum which is the finer of the two and this looks very fine to me.
A common species on calcareous rocks, walls and mortar and quite frequent around the rocky coasts of Dorset.Collema cristatum: crustose lichen