Depends on the extent of the what's behind it.
Containment, negative air, removal of drywall and any insulation, HEPA vac whole area, possible sanding of boards behind it, sealing of boards, wet clean/wipe down walls, floors, horizontal surfaces inside containment. HEPA vac again. Replace drywall and insulation then install an exhaust fan to help prevent to problem in the future.
Or you could do something peroxide based and spray some BIN or Kilz. But if the back of the drywall is affected, you're liable in the future.
It is rare. Only in flooded house. Even a very old home where ventilation is really bad, you'll see mold every where even on the driest day.I see very very little mold and showers around here dude or ultra dry climate as actually quite surprised to see it in this house I would bet that it'll wipe right off a little bleach or peroxide that the prior owner was not opening the vent or running the window
¯\_(ツ)_/¯
Your call, dude.
I was really just hoping for a video of you quasimodoing up in an attic.
what's your fix?