can't `xkill` a "Loading…" bubble/growlbox

I have no idea what program caused this "Loading..." growl notification / bubble / shoutbox / popup

but it is still there outside of the web browser. (Even though it looks like a Javascript / web-browsy gif.)

I cannot xkill it. Is there another program I can use to identify elements that are drawn on the screen, and perhaps what pid to kill?