I have a DP-60L as well. No stains on the arm. The arm is anodized aluminum some of the early anodizing had a problem with UV light and adheasion. There also could be a clearcoat over the anodizing. If stained between coatings it cannot be cleaned. It should not affect the playing.
Since you have a Denon DP-60L Does your power cord plug into the turntable?
Since you have a Denon DP-60L Does your power cord plug into the turntable?