Bristleworms (Polychaetes) & Fire Worms (Eurythoe Complanta)
Bristleworms range in size from small (about 1" long) up to very large at about 20" in length. Most small ones are an orange color, sometime appearing two-tone in color. Larger bristleworms are frequently gray or brownish in color. Bristleworms are composed of many segments and have bristles (setae) which extend from both sides of its body along its entire length, hence their common name. Bristle worms live in the sand or within the live rock. They are nocturnal and not usually seen during the day.

Good or Bad?:
Bristleworms can be considered to be bad as they can attack and eat clams, anemones, corals and even fish. But they maybe actually be scavenging and not killing these things, so in that case they're good. So it's very debatable if they're good or bad. Large bristleworms can be aggressive predators, though many large bristleworms seem to fall into the harmless scavenger category.

Capturing large bristleworms can be difficult. They are secretive and primarily nocturnal. Large ones should not be captured by hand due to their sharp pincher teeth and setae which can puncture the skin. Setting up a trap, with a piece of shrimp or food, can be used as bait. The next day the trap can be removed and the worms taken out. You can also remove them from rocks by using tweezers. Are number of fish will also eat them, including a number of different Wrasses, Copperbanded Butterflyfish, Dottyback, Basslet, Banded Coral Shrimp and Arrow Crabs. If you get some of the setae into you hands or fingers that can hurt quite a bite and even when you remove them your hand or fingers can be swallow for a couple of days afterwards.. If this does happen it's best to take a tweeter and pull them out, also wiping with alcohol helps.