That would be ACTIVE yeast from your UNPASTEURIZED beer. More than likely that beer was sitting around for awhile, or it was kept somewhere at room temp before you purchased it.
Nothing to worry about. Now if you see a bunch of it in the bottle, and looks like little wormies floating around, you might want to pass on it.
EDIT: Just Googled Bell's... Yuck, it's brewed in Michigan. Nothing good as ever come out of there.