This spray seemed to be there throughout the entirety of the Stage 2 burn but was especially visible once Stage 2 cutoff had happened.
As that nozzle uses radiant cooling, it can't be cooling fluid, so what could it be?
The thing circled in the photo is a LOX vent. The puffy white snow-like stuff is solid oxygen. It was described at T+1.05:25 in the Iridium-6/GRACE-FO webcast.