The mortar /grout between the blocks needs to be cleaned and sealed, I use a 3% hydrogen peroxide and water solution to take care of mold.
Once cleaned I would recommend a sealer be applied any quality tile/grout sealer will keep this from happening or reduce it for several years.
What’s happening, the rough surface catches dust, soap and fine debris, being in a moist or damp area mold starts growing. There are many products on the market at your grocery store that will help with mold removal but cleaning and sealing is the first step.
The next step is to make sure during/ after showering the exhaust fan is running this will reduce the moisture in the bathroom. In my area a 15 minute timer for the extractor fan in the bathroom has been required for a few years. When the lights are turned on the fan starts and keep running for 15 minutes after the lights go out. With new tightly built homes this significantly reduces the possibility of mold growth in the Pacific Northwest and will help anywhere.