Try putting white vinegar into a spray bottle and mist the mattress with white vinegar. Don't overdo it. Wipe up any excess vinegar with a towel, let it dry. Once dry, the vinegar smell should subside and hopefully the mildew smell is gone to.
If that doesn't work, you may want to try doing the same thing with diluted bleach water. Dilute 2 1/2 Tablespoons of bleach per 1 gallon of water. Open windows for ventilation. Mist the mattress, wipe up excess, and let try completely.
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