Dry Corrosion?

[Springerrr]

Hi Wise Ones,

I was shutting down the house when I noticed this stuff on the shut-off valve. You can see in one of the pictures that some of it fell on a suitcase below, but there was no evidence that any water was involved.

Could anyone diagnose?

S

 

[frodo]

that is water leaking. an itty bitty leak,

 

[johnjh2o]

Snug the packing nut.