What Causes A Dark Spot On The Upper Arm?

I have a nearly flat, slightly dark, roundish spot on the outside of my upper arm, about a half-centimeter in diameter. It has a very slightly raised, dryish texture with no tenderness. It has been present for several years without apparent change. In the last couple of years, its color has lightened and it sometimes peels off as a single thin flake with light pressure or after scrubbing in the shower and the like. The skin underneath does not generally appear remarkable. Within around a week the scale reappears. Is this simply a flat wart in an odd place or are there other possibilities?