The first time that I saw this I thought that I was in trouble. After very close examination I found that it was CO2 releasing in very, very small bubbles. It happens now consistantly for me when I brew alts. I use a secondary and keep them for a couple weeks at about 55 to 60F (kind of a lager). I move the secondary to a warmer spot (65 to 68F) about 12 hours before bottling and it is at that point that I now have noted seeing the same very small bubbles pretty much every time. There is no off smell or taste at the time of the bottling and the beer turns out fine. I have come to believe that the bubbles appear as result of the movement and a sort of diacetal rest resulting from the temp increase. Obviously I can't be sure what you are seeing is the same thing but it looks very similar and it is my best guess IF everything else seems good.