I found this when I tore out my bathroom; if the mold/mildew keeps appearing regardless of how well you're cleaning it, more than likely, it has spread throughout a good portion of the walls behind the surround. I caulked and re-caulked and it kept coming back. You're really not going to know the extent of the spreading until the surround is pulled. That stuff is no joke and too dangerous to your health to not fully investigate.