How do I get rid of this MOLD? Picture included.

[Andrew Jones]

This is in a property I just bought and I was aware of the problem and have a few ideas how to fix it, but would appreciate your input. This mold is in a bathroom with no windows so the vent is supposed to turn on automatically when the light is on, but the vent is not working and I will have it fixed. The picture captures the extent of the mold that is visible. How much do you think it would cost to get this fixed if I hire out all the work?