Rust kills bearings, and sometimes also the magnets (neodymium magnets are super reactive and prone to corrosion, if something scratches/damages the shiny silver protective coating).
Take your motors apart and clean them with some rust protective lubricant (actual lubricant, not just some water displacer like WD40). If you fly (or crash) in dusty environments, don't leave excess oil in the motors, as it might collect dirt. Spraying it and then wiping everything down (leaving just an oil film) is generally enough.