I second nudibranch. A proper identification would require seeing it's other side with decent quality. I would take it out and put it into a small container under white light to take proper pictures.
Most nudis won't survive in a reef tank for long as they have a highly specialized diets that are hard to provide. Some eat certain corals which can be problematic (or helpful in case of Berghia nudis), but I can't tell you the species of the one in your video.
It doesn't look like Berghia to me, but the quality is low and without seeing the back it's hard to tell.