Bought a townhouse in NC in November. Smelled mildew, not strongly, but enough. Last month I saw this brown molasses like, in color and consistency, coming out of the overflow drain. I pulled the cover off and saw what you see in the photo. The shower drain was coated with it, as well as behind the shower valve round face plate. I took off what I could and used some bleach to clean this parts.
The shower/tub is a fiberglass surround, put up by "cost effective" developer. God knows what's behind the surround. The largest wall the surround interfaces with was very warm from the heat in the attic. We had some attic roof fans put in that that seems to help. The closest exhaust fan, which is very weak, is about 5' away in the toilet stall. It will be replaced. Because of the roof life and HOA putting in another exhaust fan by the shower isn't a consideration.
My biggest feat, since I started smelling a hint of mildew again, is that this goo is behind the surround. There also is no access to the shower valve so I'm going to have access established, then maybe I can see better.
Any help will be appreciated.