Any chance you could get a clear pic of it's head and/or some closer up, full length pics of the worm under white lighting?
As is, I can tell you that it is a bristleworm (Polychaete) of some variety - the bristles are hard to see in the video, but you can see them if you look close enough (in fact, a few are visible at the very end of the video, so if you watch it in the browser here, it'll freeze on a frame where you can see some of the bristles; it's white bristles on white rock though) - and that it has cephalic tentacles (a fancy way of saying it has tentacles on it's head/face). Given the tentacles, my first thought would be a worm from the taxonomic Order Eunicida, from either the Eunicidae or Dorvilleidae Family.
Generally speaking, Dorvilleids are harmless/beneficial. Eunicids, on the other hand, can be either harmless/beneficial, mildly risky (think opportunistic predators eating things like snails, small crustaceans, fish larvae, etc. when starved or when the prey is weak enough), or outright predatory. Some are predators while others are literally herbivores (fun fact, even some of the predators - such as the infamous bobbit worm - will still take algae when given the chance). With that in mind, many (possibly most) Eunicids are also harmless/beneficial, so even if it's a Eunicid, it may not be a bad thing.
TLDR; it's probably harmless, but closer up pics (particularly of the head and tail) under white light may help with an ID.
For anyone who's curious, the article below is a fantastic resource on Polychaetes (the supplementary material that details family by family diet, habitat, and more is especially good IMO, though it is 340 pages long - I haven't gotten through all of it yet):
www.reef2reef.com
