Use a vapor steam cleaner to clean the mattress thoroughly, taking your time to sanitize the mattress.
If there is mold present in the mattress, there is a possibility that mold is in the same area as the mattress and it would be wise to clean the entire room at the same time.
Use an air purifier or dehumidifier to clean and/or control the humidity in the room. Cleansing the air of mold spores, as well as controlling the amount of moisture that is in the air, will prevent mold growth.
Remove the mattress from the bed frame.
Take the mattress outside to a sunny location.
Allow the mattress to sit in the sun for as long as possible. This will provide an environment in which the mattress can dry out and be refreshed.
Once the mattress is dried out, vacuum it completely to remove any mold spores that may still be present in the mattress.
Citrus and Soda
Use a citrus cleaning product to clean the mattress. Do not saturate the mattress, as this will only add to the problem. However, using a minimal amount of citrus cleaner and a rag to blot up the excess will help draw out the mold from the mattress.
Allow the cleaner to dry. You can use a fan to speed up the drying process.
Sprinkle baking soda on the mattress. The baking soda will absorb dampness.