This is silver dust (yes he's missing his cutie mark so just imagine there's a golden pocket watch there, he's also missing red splotches on the back of his neck and his back near his neck)
He's the spirit of a unicorn who was falsely executed for murder. he is over 1030 years old and can only be seen by certain animals or if he tampers with your dreams. He holds a grudge against Luna as she was his executer and each day he is reminded of this because of the large gash on his neck that's still bleeding (Hence the red splotches)
Strange thing about this guy I based him off a real paranormal experience I had where a ghost messed with my dreams as a way to talk to me (I swear I saw the ghost when I woke up) and so far that has happened twice.