In Order of the Phoenix, when Harry forced Voldemort out from the possession, what was the dust floating around Harry, and what was it called?
It’s likely just a visual effect for the movie.
The black ‘dust’ that leaves Harry is never given a name, and its significance (if it has any) is never started. The ‘dust’ leaving Harry only happens in the movie adaptation of Order of the Phoenix, and it likely is meant to be a visual representation of the Dark Lord being forced out of possessing Harry.
The book offers no more information, since this doesn’t happen in it. In the book, when the Dark Lord is forced to stop possessing Harry, there’s no visual effect on Harry - no dust or wisp of smoke leaves him, he’s just no longer possessed.
“If death is nothing, Dumbledore, kill the boy …’
Let the pain stop, thought Harry … let him kill us … end it, Dumbledore … death is nothing compared to this …
And I’ll see Sirius again …
And as Harry’s heart filled with emotion, the creature’s coils loosened, the pain was gone; Harry was lying face down on the floor, his glasses gone, shivering as though he lay upon ice, not wood … And there were voices echoing through the hall, more voices than there should have been … Harry opened his eyes, saw his glasses lying by the heel of the headless statue that had been guarding him, but which now lay flat on its back, cracked and immobile.”
- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)
With no additional information about the ‘dust’s’ name or significance, the most logical conclusion is that it was meant to be a visual representation of the Dark Lord being forced out of possessing Harry as a way of showing it onscreen.