Yep, that's yeast. You see the same thing when your yeast is getting ready to kick into high gear in the primary.
When you rack to the secondary, the yeast often isn't quite done with the beer. Racking rouses the yeast, so you get a small, secondary fermentation. That is just the yeast finishing its job (or its supper, however you want to look at it).
If you are really that concerned about preventing the phenomenon, then rouse your yeast a couple times in the primary before racking, so that everything finishes up in the primary. However, what you see is part of the function of a secondary, so I would not do anything else to prevent it.
Who said that brewing was supposed to be pretty, anyhow?
TL
Linear Mode
