@Sunni85,
The mold may be occurring because of an outside entry point of moisture or possibly from its close proximity to the radiator and its valve and the moisture it releases.
Wherever it is coming from, contact the landlord/owner/manager and if they don't or won't do something, hopefully there is a department of health and a department of buildings in your town or city which can help.
In all cases, start keeping documentation of all communications.