The marks you are experiencing are the result of a build-up of milk proteins over the heating element. To ensure your Aeroccino works optimally, we suggest removing it. We recommend the following steps: - Use cool water and a Teflon-approved (or non-stick) scourer on the inside surface of your Aeroccino to remove the patches of milk residue - Clean the inside of your Aeroccino, as above, after every use to ensure it remains free from permanent marks and scratches. You can find instructions and video guidance on the Machine Assistance page here . If you have any further questions about your Aeroccino, you can contact Nespresso directly via our toll free number 1800 623 033 (toll-free, seven days a week).