Once an object gets musty/moldy smelling, there isn't much you can do. You can try putting it out in bright sunight a few hours on each side. It will freshen it, but not for very long. Had the same problem with a mattress and a chair pad up at our camp. Ended up buying a new foam mattress with a washable cotton cover and chucking the old one. Waiting to put it outside till it is freezing cold and sunny is good, too. Jo