Most of my older equipment has this. I've heard it's oil, I've heard it's just natural fading, or even just the coloring in the resin bleeding. I don't know which, but from what I've noticed it doesn't really affect the ball any. At least for me.

I have seen two misfits that were stored in a bag at room temperature "absorb" black dye right out of the DV8 microfiber towel.  I tried acetone, resurfacing, and a few other things without luck.

Both towels (gray & black) left black on the ball that, as of yet, I haven't been able to get out.  It hasn't hurt the ball one bit, except for the unwanted dye that now seems permanent... Keep your towel off the ball when storing it is all I can say.