I think it is some kind of worm. It is very fast, the moment i turn the lights off i it retreats back to a rock. However, it is not attached to the rocks, because i have spotted it all over the tank. Im not sure if this thing i bad or what?
....could you take it out of the tank and photograph it in a separate dish; I'm having a terrible time seeing anything (or maybe it's because I had an enjoyable evening, meal and drink).
Without a better res pic I can't tell you whether it's a peanut worm (Sipunculida), sabellid polychaete worm, priapulid worm, nemertean worm, or some other 'variety' of worm (of which there are many). If it is any of the aforementioned 'worm' types then it poses no threat to your octopus.
If you can catch it! I dunno either, but I suspect Sipunculan as well. I got to play with a few in College, and it seems to resemble that. But, then again, there's lots of worms I DIDN'T get to play with, too...sigh. I never even got bit by glycera, like my prof did. I'm so deprived.