You may be aware in January of 2016 a new law went into effect regarding mold in property. The new law basically says that 'if mold is found in the property then it has to be addressed or fixed by persons having the proper credentials'. That is paraphrased from the actual text of the published law for California. For the sake of argument I will be just briefly discussing how this relates to rental properties.
According to California law mold being present in rental properties can make it substandard housing which requires the owners to fix the problem. This, however, does not relieve the tenants of their implied responsibility of performing regular cleaning maintenance of their home such as 'cleaning the bathroom every week' so mold does not start to grow on bathtubs and sinks.
I have received several calls this past year from renters concerned about mold in their home. I have inspected homes where the landlord wants 'to bury his or her head in the sand' and ignore the problem of apparent leaks or moisture intrusion thinking these will go away by themselves. Or after having a leak repaired fail to have the area dried properly believing that it will dry on its own in a few days. That does not happen like some property owners believe or property management companies believe, necessarily. The liability increases tremendously for landlords if they fail to do their 'due diligence' in this area.
The water damage industry (including mold remediation) has developed and established industry protocols to dry water damaged materials and remove mold over the last 30 plus years. When moisture intrusion occurs (whether from a plumbing leak or a leaky window, etc.) it does not necessarily dry out on its own until it has helped grow a fair bit of mold. By that time a larger problem has developed and can cost a lot more to fix along with potential liability for the owner and even affect occupant's health. This is one reason why it is necessary to dry the affected areas promptly so mold does not grow and moisture does not continue to damage property.
Not just any contractor can do this type of work. It needs to be someone trained specifically to do it per established industry guidelines. If a person has not been trained how to do it the problem will likely become worse and unnecessarily expose occupants to an unhealthy situation.