I've had three of these worms in my tank this week. They stay on the rocks, they get really long, and they're fast. When I suck them in, they shrink a lot.
I tried Google, and I think maybe it's a horsehair worm?
yeah - bad pic because it is in my turkey baster - they're long, white, stringy - almost looks like some sort of parasitic worm you'd find around your dogs fecal matter. When it's on the rocks it's almost blended into the rock, whitish but semi-transparent. here it seems to be grabbing onto the turkey baster on the top right side, it then elongated 5x what its body size appeared to be trying to find a way out. I've no clue what they are, what may be causing them... if they're helpful or harmful - so I've been removing them all together.
I'm considering getting a wrasse to hopefully eliminate them, but I've also got several hair worms that I actually do enjoy having in the tank lol.
I was thinking that it might have been a horse hair worm, but looking at horsehair worms more closely - they appear to be much more white with no transparency compared to this thing: ADW: nematomorpha0070.jpg