Just found this when the lights came on in my chaetomorpha tank this morning. He cruised around the glass for a few minutes, then slithered off into the chaetomorpha and out of sight.

Kind of reminds me of a roundworm, about 2" long, no discernible head.

Never been good with worms. They all look like....worms to me.

http://www.chucksaddiction.com/hitchworms.html <--- check there

might be some sort of tropical ribbonworm...

Hmm, I skimmed through chuck's addiction and melevs reef and couldn't find anything similar. Whatever it is, there's nothing in the tank for it to really harm, unless it eats feather dusters. I keep a small damsel in the tank, so in the end whatever it is, it may just wind up on the fish's menu...