Update- actually, I think it isn't mercury. The lamp end got a slight teardrop shape. This lamp also seems to have a loose connection, so maybe the cfl ballast is striking it on hot cathode, but the arc remains when the lamp is in cold cathode mode (lamp is shaken and connection loosens). But I don't think this is the case, because my other lamp is also experiencing this on this ballast.
Also, in the picture, the spot isn't the filament structure, as it wasn't there before I lit it on this ballast. The lamp hasn't been running on it for more than 15 seconds.