Yes it doesn't look like bare aluminum, so looking at it, must have some kind of coating, but it's all the same even on the inside of mod. Looking closely at the tiny scratches on bottom, that part is slightly shinierIf they are finished in the raw, look like bare metal. Then they may have just been clear coated. Paint or clear comes off best at edges and of course the bottom since it has contact with other surfaces.
Could be the mod reacting with cleaners on couch or possibly sweat/oils from your hands causing the reaction to start.
EDIT: Basically oxidation/tarnishing