Looks like a reflection of the interior ceiling lights...Not from that angle. (Says the optical engineer after doing some quickie ray tracing in his head.) For that to be something inside the cabin, the source would have to be in the seat back in front of photographer.
Old airline passenger me says there are TWO surfaces doing the reflecting ~ the inner safety screen ~ it's plastic, and has a different index of refraction than the outer window ~ it's super duper glass. Think hard and you can unbundle the two images in your mind.
You'd be able to see this from the A seat looking toward the F window seat