If not for the rapid sliding and rotating movement, I would have thought it was just a bit of detritus stuck amongst the inevitable dust. However, the other side looked like this (at high magnification, greater than life size on the camera sensor, cropped to get closer still, and many times life size on your computer monitor):
A little internet searching revealed that it is the larval stage of a common variety of Lacewing. Many of them carry a backpack of fibers like this; my guess is that it's a camouflage against predators. This one was quite systematically scouring the glass surface for tinier things to eat. I will have to make a point of photographing some adult Lacewings this summer.
[Canon EF 100mm f/2.8 L IS macro, 37mm of extension tubes, remote flash]