I have the Dr browns one and it does this. It’s normal just clean it off after each use
I use lemon juice and then pour boiling hot water leave it for 30 mins and clean, looks brand new, I do it once a day
You need to descale it after every use. Use normal dishwashing liquid to wash after every use. For now use baking soda and lemon to descale it completely.
I don’t have this steriliser but do have the electric steamer one. My metal plate does the same and I believe it’s due to the water in your area. We have hard water so I have to clean it every day. I use either lemon juice or white vinegar. Just put some on the plate as you would the water and heat for 30 seconds, it should wipe right off and remove anything burnt or those brown bits too x