Problem With Sliver Dollar?

[Carl the Crimson Wonder]

Ok my silver dollar has a spot on his back fin. It looks almost as if a piece of black gravel, or like a (small black snail shell of some sort) is stuck to his fin. He doesn't seem to be bother him, but I was wondering what this was and how to fix it (If possible). My other silver dollars and other fish are fine. My water perameters are 10-15 ppm nitrAte, 0 ppm nitrIte, 0 ppm ammonia.