Great question — answering as the UphillTech team:
This is mostly a cosmetic issue. Incoming cold water in the pipes creates condensation on the pipes during humid weather. Gypsum drywall is mostly hydrated calcium sulfate -- chemically a "salt" -- which absorbs and retains moisture. Gypsum dust and debris on the steel parts of the valve will promote rust on steel about as well as table salt. To clean this up, first clean up the drywall hack job by finishing, sealing or painting the exposed gypsum edge in the vicinity of the valve so it stops dropping particles on the valve. If you wish to make the valve look better, remove the nut and handle, wire-brush any rust on the valve body, thoroughly clean the valve body to remove any remaining...
Full answer by MTA: https://diy.stackexchange.com/a/331935 (CC BY-SA 4.0).
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