Had the same problem on my VW Passat, but it was more visible than yours and when you touch the glass the liquid moves inside. Yours looks more like a design like everyone said. I never bothered fixing mine though.

 

Taken from a google search:-

"An electrochromic rear-view mirror typically consists of an electrochromic gel placed between two pieces of glass, both of which have a transparent coating that conducts electricity. In addition, one piece has a reflector. According to Gentex Corporation, the mirror system includes: a sensor that faces towards the front of the car, sensitive to low light levels, which signals the mirror to detect signs of glare; and a second sensor that faces towards the rear of the car and picks up glare from vehicles behind the car."

Obviously an internal defect,either the coating or the gel.Mirror will need replacing unless you can live with it.