[geekmaster]

Originally Posted by HarryT

It looks like a burst "capsule" on the eInk screen. Basically, the liquid has escaped from one the capsules inside the screen which display the image. There's no fix, other than to replace the screen (which isn't economic to do).

If anybody cares about technical details:

Spoiler:

Each pixel is composed of many of those capsules (called "beads" in some of the eink documentation). Each bead is filled with oil and many tiny black and white oppositely charged particles. One type of eink used zinc oxide for the white particles. An electric field placed across the beads causes the particles to move to the front and back surfaces depending on their color. An alternating current waveform is used to minimize artifacts of the electric field affecting nearby pixels other than the addressed pixel, and also to create various shades of gray with mixtures of black and white particles. Because of that, you can see a (silvery) negative image on the back surface of the eink glass if you remove it from its protective back support.

If a bead is damaged, the oil can leak around adjacent beads, carrying its colored particles with it, resulting in that splotch as shown above.

EDIT: The oil in the beads also explains why the eink displays are slower with more ghosting when they are cold (higher viscosity). The ideal operating temperature is 25C/78F, but there is some degree of temperature compensation in the eink firmware.