Your mold is caused by too much humidity and not enough ventilation.
Most likely you will have to repaint. You could try cleaning, but it will leave stains.
Before you repaint again, wipe the walls down with a 15% laundry Bleach to water solution. That will kill the existing mold. Allow the bathroom to dry thouroughly before repainting.
In order to keep the mold from returning in the future, you will have to add a combination of heat and/or ventilation to dry the moisture between showers, and other water usages.
This usually is as simple as keeping the door open to the bathroom when not in use, turning on the fan more often, opening windows during good weather, and keeping the heat on in the bathroom during cold months. You may also want to consider installing a heat lamp ceiling fixture.