Bathroom mold, explained
Surface Spots or Something Behind the Tile in Your Rapid Valley Bathroom?
Bathroom calls split into two groups. Some of it is surface growth on grout lines and caulk, feeding on body oils and soap scum, and a good cleaning handles it. The harder cases are growing out of sight, in the wall behind the tub surround or in the subfloor, kept damp by a cracked shower pan liner, a dripping valve body or a toilet seal that's been seeping for a long time.
People fight caulk mold for years without winning. When the growth is inside the bead, no spray will reach it; the old silicone gets sliced out with a razor, the gap is dried and treated, and new caulk goes in. Overhead, the ceiling above the shower collects condensation when the fan is too small, gets shut off right away, or blows into the attic rather than through an exterior cap.
In a lot of older baths, the tile was set over standard paper-faced drywall instead of cement board, and that facing feeds mold as soon as moisture reaches it. When the cause is a leak, the remediator and a plumber usually have to coordinate.