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This has been a bit of a mystery to me for a few years. Thought it was Reboulia at first but looked kinda wrong. @guidobrusa first suggested Plagiochasma which I was unfamiliar with as it doesn't occur in Florida and suggested further study, which I've been doing off and on the last few years, finally getting some good samples this last December.

I think what is the "smoking gun" are the circular ventral scale appendages, if I'm interpreting them correctly. Also, as @guidobrusa pointed out early on, the carphephores are ventral and not terminal.

Was not certain about P. wrightii vs. P. crenulatum, the appendages were more or less 15-16 cells in width on average which is right in between but the thallus width was about 5mm which is closer to the former, as well as the carpocephala which I didn't observe in pairs.

Used this key below
https://www.mobot.org/plantscience/BFNA/V3/Aytoniaceae.htm

Photo 5: partial ventral scale with appendage

On dead juniper wood. Couldn't locate any on living limbs. In a Mississippi River bluff remnant cedar glade, just over the edge of the bluff.
Cortex K+y, medulla C+r.

Chemistry, scrobiculate upper surface, fine soredia, apparent pruina on lobes, and brown underside indicates Punctelia perreticulata.

Growing in a lower lying area that stays wet longer than the surrounding brushland and is a few feet below elevation on a nearly undetectable slope. Growing with Manfreda longiflora, Mammillaria heyderi, Echinocactus texensis. Many dead mesquites, living ones extremely dwarfed likely due to salinity