If this is the first time you used it, you would have been wise to clean it thoroughly with an alkaline cleaner like Oxyclean or dishwasher detergent in hot water to remove any grease, oil or dirt that was present. Then a rinse with vinegar in water to neutralize and dissolve any excess oxidation present. After the first use, the wort will dissolve or reduce the oxides of copper and give you a shiny coppery chiller. No need to clean it after that unless it sits a long time and corrodes again to brown or green colors. If it does, a water and vinegar soak will take care of it.