Wide finger mark on front camera lense

I'm opening this new subject because I have this wide stain of the camera lens on my 6+ (pics attached below), it appears both on the screen of the phone when I take a picture and also on the final picture that I transfer to my laptop. It is about the size of a finger mark. Tried to clean the lense with a microfiber cloth but it does not go away. Any idea on what could have caused this stain and how to remove it? When I look at the lens it looks immaculate. The front camera is working OK, and the screen doesn't show this stain when I'm not using the camera. I also removed the case and it made no change. Thank you!