HERE IS THE BAD ADVICE!
Grimy bathroom walls are a major red flag to buyers.
Here is an easy way to get rid of surface mold: Mix a spray bottle with one part water and one part bleach. Just spray it on the wall, and watch the mold disappear. Give it a fresh coat of paint, and your grimy bathroom will go from red flag to red-hot.
THIS IS TERRIBLE ADVICE FOR HOMEOWNERS!!
If they have mould, they should have an expert come in and determine what's causing the mould and remedy the underlying issue. Even if it is just surface mould (which the majority of homeowners wouldn't be able to determine by themselves), bleach is not the right product to use to clean it off. There are several other products specifically meant for this type of application, even a couple at Home Depot for homeowner use.