Mildew can cause a musty smell. To avoid that problem, try to get rid it using bleaching powders. You can put it on the places that have musty smell like basements etc. Then try to scrub your bathroom with bleach and rinse it with water. Then wipe the dampness so that it wouldn’t create mildew again. Open the windows when you do this so that the smell will be gone in less time possible. When fighting mildew, always keep all things clean and in case you used your shower, bathtub and even washing machines, dry them after. Is this toxic mold? Find the right resources online. If you have exhaust fan you can use it once in a while to really make the air in your house dry and take away that damp and musty smells.