Why do you think it is drywall?
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How accurate is your moisture meter? I do not think I have ever seen 0.0 & 0.5 moisture readings before.
Gypsum is not a man made material.
Like others have asked, how did you determine that it is drywall? If you saw the back side of it in the attic, it could be rock lath with plaster on the exposed side. This scenario would make sense where you are seeing efflorescence.
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My take:
- The AHU (Air Handler Unit) is above in the attic.
- at some point in time there was a water leak or high humidity condensation from the water pan that was filled;
- Thar humidity or dripping migrated down to the ceiling.
- Ceiling may be plaster material with some aggregate that reacted with the water particles and generated a material expansion due to segregation of some salty derívate…
This “theory” could only be confirmed by a sample extraction taken to a underwriter’s chemical laboratory for further analysis.
I would recommend (in my Report!) to have a competent certified HVAC Contractor or some FBI Agency to further study this situation. May be replacing the ceiling materials with some most updated drywall material and reassuring that the AHU pan is well/properly drained outside to prevent future deterioration of the ceiling.
I know this is a crazy guess, but I am open to feedback…!!
It’s enlightening to see that so many have learned to spell efflorescence. But the fact that the patch of ceiling is directly under the A/C unit and drain pan, and the affected surface malady has developed a florid pattern, would suggest the hypothesis of fungal growth ( maybe even such as Sclerotinia sclerotiorum) is more appropriate.
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LOL! Isn’t THAT the truth.