My first method of attack is to wipe down the inside of the case with either a mild soap solution, or Lysol Disinfecting Wipe, neither of which should damage the case at all. If the smell persists I use a product called Concrobium Mold Control which kills mold and is supposed to prevent further growth. You didn't mention where you live, so if these products are not available locally I'm sure that there are comparable ones you could use. Keep in mind that if the case has mold, the machine usually does too. Mold loves ribbons and any felt/sound dampening material that a machine might have. With a Corona 3 you would only have to worry about the ribbon.