Water Stain found during inspection - looking for suggestions

[Jessie Xu]

I'm in the processing of purchasing a house and the inspection found water stains on the wall between the master bedroom and the bathroom next to it. The inspector confirmed that no active leaking was found and it's not wet. So this must be a leak happened before. However, nothing is mentioned in the disclosure. I've attached the photo:

The inspector mentioned that it's hard to evaluate the damage inside without opening the drywall. I don't think seller will agree to open the wall just to do the inspection. Given that it's no longer an active leak, should I worry about it or should I walk away from this house?