When an angle grinder is used nearby (on steel
, obviously) the sparks, which are moving at close to 200mph and are white hot, can hit a surface hard enough to slightly embed themselves - I've seen car windscreens pitted by sparks. Welding nearby has the same effect but worse.
Even though a surface like FG is later painted the particles can eventually bleed rust through.
I can't see well enough from here to be sure
but that's what it looks like to me. You might get away with sanding
until all trace of staining is gone and then repainting.