Ignoring a water leak in your home is very dangerous, as this can allow mold to form on building materials and, in turn, mold spores could become airborne in your home. These spores are very dangerous when inhaled; that mold is also typically expensive and difficult to clean up. A water leak can also soften drywall and flooring materials so they need to be replaced, mold or not. Since you can't see water leaks behind walls, note a few signs that you might want to call a plumber and have the home's pipes checked for damage.
You may know what old, musty water smells like, and you shouldn't notice that smell in your bathroom. When you smell mildew in any area of the home, you need to have it checked out as soon as possible, but when it's in the bathroom, chances are there is a drip or leak behind the walls and the water has formed a puddle or somehow collected onto another pipe or flooring. Rather than being absorbed by the home's drywall, you now have a small collection of old water that is allowing mildew to form. If you can smell this in the bathroom, especially after you've allowed shower steam to completely clear out and have cleaned the tub and tile of any standing water, you should have a plumber check this odor.
Standard steam from a shower shouldn't bubble paint, peel wallpaper, or otherwise damage the finish on your bathroom's walls. This steam is usually not thick enough and doesn't sit on the finish long enough to break down the glues and adhesions in paint or behind wallpaper. However, a water leak will mean a continuous flow of water in that area and allowing the water to collect on paint and behind wallpaper. In turn, those adhesives may break down and you might see bubbles or peeling.
Mold in opposite corners
Mold in the bathtub corners is not unusual in a bathroom as not all water in tub may evaporate before mold can form, but when you see mold in the opposite corners of the bathroom, this can signal a water leak. This usually includes mold along the corners of walls behind the bathroom door where there is little light and air circulation, or you may see mold form along the ceiling corners opposite the bathtub. This mold is usually forming from water behind the walls, not the steam from the shower.