I mean, I can see how it would superficially look like a very small boom mic, but that ain't a boom mic. My best guess would be some kind of little paperclip type thing, maybe used to hold the cels to something? I can see why people think it's a little creepy. It's weird, but not unheard of for stuff like that to not be caught during production, especially if the budget was low and the deadline was rapidly approaching. Though, the movement is rather unnatural to have just been shifted between filming cels. Weird.
Unless it was explicitly pointed out, I never would have noticed this. Might be a hot take, but IV is among my least favorite sequels, so I haven't watched it nearly enough to notice little things like that. It's interesting though, and I wonder if anyone who worked on the movie would know.