Bathroom mold: surface cleaning, or something behind the tile?

Usually a cleaning job

The EPA notes that always-damp places like showers are hard to keep completely mold-free, and that more ventilation (running a fan or opening a window) plus more frequent cleaning usually keeps recurring mold to a minimum. Hard surfaces can be scrubbed with detergent and water and dried.

Signs it's inside the wall or floor

• Mold that comes back within days on caulk, grout, or the ceiling.
• Soft, swollen, or stained drywall next to the tub, shower, or toilet.
• Loose or cracked tile, or gaps in the shower pan.
• A musty smell under the sink or behind the vanity.
• Water stains on the ceiling below an upstairs bathroom.

Those usually point to a leak or a failed shower pan, which means opening the wall or floor, removing wet materials under containment, drying, and repairing the source.

Preventing it

• Run a bath fan that vents outside during and after showers. The EPA recommends venting moisture-producing appliances outside where possible.
• Keep indoor humidity below 60 percent, ideally 30 to 50 percent, per EPA guidance.
• Fix drips and re-caulk failed joints promptly. Don't paint or caulk over mold; clean and dry first.