Aluminium oxidisation) can be removed with cotton wool dabbed with a little vinegar. Thereafter, fairly regular (depending upon usage) cleaning with soapy water and thorough drying should keep your ROK in excellent condition. Full cleaning instructions can be found under the FAQ section of our website.
If the vinegar does not work, I would suggest trying a non scratch metal cleaner to remove the oxidisation followed by a metal polish. A good hardware store should be able to help you find a suitable non-scratching cleaner.