Are there any bathroom fixtures on or near the common wall? Like a sink, tub or commode? Other option would be termites? They can get into the center of a home through cracks in foundation, they typically will be in walls with moisture damage. I’d suggest having a licensed plumber inspect the areas for you to pinpoint what all is going on to help get it resolved.

Are there gutters/downspouts outside that corner of the house? Looks like moisture penetration. Check to see if water is pooling up to the foundation in heavy rains too. Need to get the exterior corrected before working on the interior.