Wild looking Green worm (probably) id'ed
Credit to Ron Shimek, PhD
He's not 100% sure, and can't be without a specimen to examine, but based upon his description (below) I'm confident that he nailed it.
Palola individuals live in reef rocks and are eunicid worms. They may get big, some specimens are over 2 m (6.6 ft) long. Each worm spends its entire life living in a burrow in the reef and extending from the burrow to feed on many things - they are sort of omnivorous predators. They eat just about anything that they can catch. The upside is that they can't catch a lot. Most mobile animals can escape. They may nibble on soft corals or sponges, though. Other eunicids get much larger than Palola, and the green color of Palola individuals is fairly distinctive.
When the time comes to spawn, the worm remodels or converts part of its body into a disposable reproductive structure. This is, in effect, a clone of itself, that buds off the adult worms body. This clone remains attached to the adult/parent until the reproductive event. It lacks the gut and most of the muscular system. The nervous system is also rudimentary. The whole interior this clone, while it is attached to the adult is filled with either eggs or sperm.
When the night of reproduction occurs, and these events are very specifically timed, all the Palola on a reef release their clones into the water. The worms tend to swim upward and writhe around a lot, often rupturing their body walls and releasing the gametes (sperm or eggs) carried within.
(once I read this I realized that the worm appeared to be breaking up...)
Fertilization occurs into the sea and eventually larvae form which find a new reef and settle to grow up to be adults in the reef rock. The adults may live for many years releasing a cloned body each year at spawning time. See here:
Palolo worm - Wikipedia, the free encyclopedia
These spawning bodies are called epitokes, or sometimes heteronereids. And a single epitoke is what I think was in your tank. The color and dimensions fit very well for some epitokes but, as I said, for confirmation I'd need a specimen. These are not uncommon in reef tanks, the worms arrive in "live rock" and will do well in some tanks. These released epitokes are harmless, and actually are good fish food (and human food, too - they are common enough during some spawning events that some island cultures harvest them as food).
Samoa Worm Sperm Spawns Annual Fiesta
