Chameleons rub their eyes frequently throughout their lives. The reason is that they do not have the flexibility or opposing thumbs we do to
properly remove debris from their eyes.
I wouldn't worry about it unless you see it often and you know that you're misting enough for the chameleon to be able to drink. I say the drinking part because chameleons will use the water they drink to swish debris out of their eyes- some magical mechanism we know to be true but aren't quite sure exactly how it works on a physiological level.
I hope this helps calm your nerves a bit!
If you find the chameleon is closing that eye or you see any tearing or discharge from that eye, you may need to aid with a saline flush. Feel free to PM me for more info on that!