I will say this again, I'm sure. Probably many times in many threads. That is (was) a bristle worm. Most likely E complanata. This is the most common species of worm in home aquariums. It rarely gets that large because we don't feed enough. But in an overfed tank, they get big.
Now, to address the few "that's a fireworm" comments I saw. Feline is to cat as fireworm is to bristleworm. In other words... Same thing. Both are generic umbrella terms encompassing a number of species. Bristleworm = fireworm. And they are GOOD things to have.
Couple specific points.
Size is not a factor in determining whether this is a "Caribbean bearded fireworm" because the cbf is nowhere near the largest species, nor is its size in any way remarkable. "Common" bristle worms grow to a max size of 1" - 3' depending on species and availability of food.
Coral eating is what all bristle worms have been accused of at some point or another. It is also not something that many do. True, the cbf does eat gorgonians, but if it were a cbf and there were no gorgonians present, it would never have reached that size.
"Kill it!!!" Worst. Possible. Advice. These are the most beneficial organisms you can have! In a recent move, my tank got cold and most of my micro fauna died. Anyone in the Tacoma Washington area who has an excess of worms, pm me because I have very few alive and I need more.
In conclusion, next worm you see in your tank, don't panic. Thank it for keeping your tank healthy.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef