Mold on the walls on the bathroom
Obviously the root problem is too much humidity in that area and fixing that properly would mean adding ventilation to get the warm and moist air outside.  Short of that, I can think of two things you might do:


Clean the area with a chlorine bleach solution (not 100% bleach) but something like 1 cup to a gallon of water.  Wear gloves as this will burn your skin.  And scrub the visible mold/mildew off.  The bleach will help to kill what's there.  Use a brush if you need to in order to get into cracks.  After that, you might treat the area with a commercial mold/mildew inhibiting product.  You should find something at a local grocery or home center.
After using the bath or shower you might place a portable fan in the room to help extract the humid air from the enclosed space.