I made the mistake of using Muc-off on the black plastic around the head of the tank and allowed it to dry before washing off - it then left a permanent white mark! Not sure you'll be able to remove it permanently but I've found the only thing that covers it (temporarily) is to wipe over it with ACF-50. I tend to wipe over most of the black surfaces anyway as it covers any similar marks, if you can stand a bit of an oily residue. I put a little on a rag so I'm not plastering it all over and it's fine.