Iphone 6 Plus - Back Casing Problem

Anyone have problems with the back casing of their IPhone 6 Plus? My phone is in a case and has been since I got it about a year ago. I took the case off to clean and I have white spots that look like dirt or dust, tried to clean them and they don't come off. Wondering what this may be and if this is something apple would consider a defect and replace. If not, is there a place to bring a phone to get a new back covering?

Thanks for any advice.