Possible Causes Nicotine Residue – Smoke stains can resurface in humid conditions. Soap & Hard Water Deposits – Mineral buildup from repeated splashes. Mold & Fungi – Some molds appear yellow in early stages. Paint Oxidation – Moisture can react with paint, causing discoloration. How to Clean It ✔ Nicotine Residue – Vinegar and baking soda help remove stains. ✔ Soap & Hard Water – Use a descaling agent or vinegar solution. ✔ Mold & Fungi – Clean with bleach or vinegar; ensure ventilation. ✔ Oxidized Paint – Repaint with a stain-blocking primer if needed. Prevention Tips ✅ Ensure good ventilation (use an exhaust fan or open windows). ✅ Wipe surfaces regularly to prevent buildup. ✅ Address mold or mildew immediately