u/littlebego

Is this mildew? Just bought a house and we think it hasn't been lived in for a while. We're hoping it's just surface level mildew, but it's in a few different rooms, is it worth cutting into the drywall, or should we just prime it with a solid stain blocker and paint it?

The wall pictures is in a decently ventilated office type room, and the guest bathroom is behind this wall. There isn't any of this staining in the bathroom. It's also in the master closet, all over the wall, not just at the bottom.