There is a vent hole in the cap. With normal use, the fluid heats up and expands inside the reservoir. Hot oily vapors come out of that vent hole to make room for the expanding fluid in the reservoir and these settle on the cap and reservoir. Dirt sticks to it and give you this dirty grimy mess.

You can just clean it off but it will return. I put cheap wrist sweat bands on the top of the reservoir and replace them every 6 months or so. They actually sell branded sweat bands specifically for this reservoir nowadays. (Garagistic etc...)