The iPhone 6 — not the 6s, just the 6 — is plagued with a manufacturing or design problem with its front-facing (or in Apple’s confusing parlance, “FaceTime” camera). Over time the camera itself will shift position inside the phone. It causes a distinct crescent moon effect:
I stopped in the Apple Store “near” me last weekend and finally got it fixed. They replaced the screen – under warranty, so there was no cost to me. If you have this problem, get yourself to an Apple Store. Make a Genius Bar appointment to save yourself time.