Pretty good picture. Seems to be a spaghetti worm, as Spanko already said.
This are another type of tube worm, unfortunately most find them one of the less beautiful. They extend their feeding tentacles much like a feather duster extends their plume. They are harmless filter feeders.
The only downside to these worms is that if they are really close to corals, the tentacles can cause the corals to not open. It won't really harm the coral per se, but obviously if it doesn't open for a long time, you can run into some problems.
Other than that, you're okay. Enjoy your worm