The nearest thing to a lichen Flora for Greece is the notes provided here. No Flora has ever been formally published. Because Greek lichens are poorly known by European standards, and much research remains to be done, it will be many years before a critical Flora can be published. In the meantime, you can regard these notes on Greek lichens (pdf) as a draft Flora. Keys to species are included.
If you use Floras from western Europe to determine collections of Greek lichens, you should proceed with caution. It has been my experience that they do not always work well. I am still unsure whether this is because there are many undescribed species in Greece, or because the full range of variation of some widespread species is not expressed in collections from western Europe. Floras from more distant geographical regions must, of course, be used with even greater circumspection.
If you would like to contribute to the effort to prepare a Greek lichen flora, please contact me. A sensible goal at present would be to prepare good descriptions, from Greek material, of species that are not yet covered by my notes. That means especially species from northen Greece.
Nearly 200 taxa of lichens or lichenicolous fungi have been described from Greece, and the status of many of them is unclear. You can find information on them here (.rtf file). Professional lichenologists may wish to take up the challenge of clarifying the application of some of the names.