Lichen

Thank you both for your ident and advice .. much appreciated : )

I agree with drP. I suggested Graphis scripta based in my experience with european lichens, but I have no experience at all with Asian lichens.
Besides, I guess that microscopy is necessary to distinguish the species.

I agree with arlanda that it is genus Graphis. There's a Catalogue of the lichens of Thailand available online at http://www.aseanbiodiversity.info/Abstra...
They list nearly 80 species that are currently or formerly in genus Graphis. G. scripta is not mentioned. The catalog does not include illustrations, but many can be found via an online image search. Many of them look similar to your spotting. I'd recommend calling it Graphis sp.