What the heck is this?

[SmallFry]

So I was peering into my reef using the flashlight on my phone - as you do at 11:30 in the evening, and I saw this thing swimming freely around. No idea what it is, and it looked like it was up to no good, so I fished it out. Looked almost bristle worm isn but I've never seen one freely swimming around. About an inch long stretched out I'd say.
Any ideas what it is and if it's benign or malevolent? (Sorry for crappy iPhone pics)




Thanks in advance,
Rob.