Definitely an internal reflection.
It's caused by the bright moon hitting the sensor and reflecting back up onto the inside of the lens, probably bouncing back off a lens filter like a UV filter if one was fitted and then returning back onto the sensor causing this anomaly.
Lens hoods help reduce this effect but removing any lens filters such as UV filters will minimise this best when there's any bright sources of light in the FOV.
Also, higher quality lenses have better multicoating to reduce internal reflections but they do still occur if the light source is strong enough.
RB
