I sincerely hope you got the set cheap, and didn't' pay going price. This wouldn't be any type of error worth a premium for. Looks like a milk spot or something detracting and damaging to the value of that coin, and the value of the set as a whole. You'd have a hard time selling it unless it was really cheap I think. it's not a lamination, it's not annealing, this is some sort of staining on the coin after the strike, in other words, a low quality mint released set, especially considering the coins finish in this set is the main selling point of it.
this isn't an error, this would be Post strike damage/staining i'd think based off these pictures. Sorry to be the bearer of bad news. Quality control should have caught that and never sent it out. I don't think it's a fingerprint either though, just a milkspot type stain from some contaminant that got onto the coin that they didn't catch on it's way into the set and out the door.