If you are talking about the tile on a shower.... you are out of luck! You can make the outside which we see look better for a short time using either bleach or steam or something like that but in reality.... the mold is growing on the BACK of the tile. The only way to get rid of it is to tear out the tile and drywall and start over. And since there are so many different ways a shower could be made, if there is wood that touches cement it must be treated wood. Non treated will cause many, many problems. And, if they didn't use the correct drywall, that can be a big problem too. Just remember.... you are a house CLEANER. You are not there to rehab the place. If they want things like that 'fixed' they will need to call someone else.