Water spot. Normal coin. Also the number one reason that is can't be a 'D' mintmark. All the mintmarks after 1990 were in the same location on all the Denver coins. So this is not a mintmark on that area. Impossible. It is a Philly coin, not a Denver coin. The affected area is level with the fields and devices, so it is not a mintmark.
I am guessing this is what the person was seeing. If look at the coin again >>> look to the right of the 6 up a lil in the date it kinda looks like a weak D mint mark!! most likely not but you can see a D