That seems most likely. (Was I misled by the Trentepohlia?) Now I'm wondering if the Arthonia is lichenicolous on the Lecanactis or is the white thallus partly A. arthoniodes and partly L. megaspora. I'll examine the specimen more closely.
Around 2am remembered lichenicolous fungi which are confined to the ascoma of their hosts. Find e.g. Arthonia intexta which occurs immersed in hymenium of Lecidella species. Need to look further into parasites of Lecanactis, and more carefully examine the black apothecia. Something like A. intexta seems the most likely explanation.