The wires that go from the ceramic cup to the stalk also heat up when you fire the coil. You've got a low amount of juice in there, and the wire is heating up enough to vaporize the juice left on the wire. I got that all the time, and it doesn't really do anything bad. The heat will make your juice turn dark, but that's about it.
If it really bothers you, and you're handy with a soldering iron, you could unsolder the wires from the stalk, slip some silicone tubing over them as a sleeve, then resolder. That would keep the vapor down to a minimum.