the ac is accumulating condensation on the metal vent louvers and duct work causing a slow deterioration of the surrounding rock.
This happens on many homes.
You can fix the problem around the vent and use rock hard instead of joint compound. Rock hard will never deteriorate.
Then the easy way is to buy oversized vents to hide the rock that is damaged. Cut away the bad rock. You will have to add plastic inserts to attach to the wall since you will be beyond the screw hole on the old duct flange.