Oily spot on baby's eye....

I've noticed this a few times now my baby has a spot that covers from the bottom of his pupil to the bottom eyelid that looks oily/rainbowish. Best way I can describe it is it looks like oil on water. Its only on his right eye. It doesn't seem to bother him at all. I know he can see, as he smiles/laughs at people, follows objects, and is always looking around the room. Just wondering if anyone knows if this is something to be concered of? has anyone else noticed this on their baby's eyes? I'm just concerned maybe its effecting his vision in that eye.