Maybe enthusiastic spraying at the dealers with either Wheel Cleaner, or Muc-Off?
Al
Clean it with WD40 and a soft cloth. If it doesn't come off take it back and insist on a replacement pipe.
What have you been cleaning it with? I'm pretty sure mine doesn't look like that.
Had it ACFed and cleaned by ALLYEARBIKER last week but then cleaned it myself using Castrol and just car cleaner. Will most likely use MUC OFF in future but not sure what to do to clean exhaust as it really looked shite until I did as suggested here with WD40. Emailed BMW to ask but nothing back yet !!!
.....I cleaned mine pretty thoroughly with Greentec at full strength last week and I've just been out to the garage to confirm there's no staining and it's as new.
Those from Yorkshire can dilute it 50/50 with water, which is a tighter biker bonus, too. Thanks for sharing.