Only saw him do it once, he's a baby about 2 months old
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!