I had what looks like a very similar situation. If you’re not that concerned about the mold in the attic area, or can’t afford a major rix/remediation you could get some Kilz Upshot. It would cover the water stain and you’d be good to go for painting over it. I didn’t need to paint as the Upshot matched my ceiling paint perfectly. Edit: You should still attempt to clean the mold off as best you can with bleach first, but even after cleaning there will likely still be some staining which you could use the Kilz to cover.