As for starting a cleaning business, I think you should have knowledge of all areas of sanitizing and removal of all germs, including mold. Mold is something that happens to any and every one from one time or another. Different situations or conditions causes mold and it is not healthy, this along makes it a plus for my company to have the knowledge of removing this fungi. 

A client may call about one service without acknowledging that the clean-up for mold is a totally different process, this is when your sales pitch comes in without sending the client to another company, offer a discount on top of the service they originally called about and get the job done.