Could be a bit of debris or possible a "scuff" from the rock. They do tend to dart a fair amount. Do a few large water changes with well aged/aerated SW
to help reduce any possible DOC
. If a mild bacterial problem, the water changes will help quite a bit. Be sure the foods you feed are also properly nutritious and/or fortified. Diet and high water quality are the main "self helpers" when it comes to common ailments.
If C. irritans
or another common parasite, you will most likely see the spot disappear to be replaced by another a few days later. Watch the patch/spot to be sure it does not get larger.
Any aggression from tank mates?