Your right about the pom pom. They are harmless. I have 2 and those are ONLY crabs now allowed in my tank except a few smaller hermits. Here is a bit of Bristleworm info I picked up from another site along the way. The mentioned would be considered an errant Bristleworm or bad boy.
Errant bristleworms actively move about in search of food, which may be</em>
other small vertebrates, algae, corals or almost any organic matter</em>
depending on species. Errant bristleworms usually resemble centipedes in</em>
general appearance, and have strong jaws.</em>
3. Errant bristleworms.</em>
The real problem childs in tanks. They are ugly, move in an unnerving</em>
manner, can pack nasty poisonous bites and/or poisonous bristles, and may</em>
eat things the aquarist would not like them to eat. In general appearance</em>
they resemble centipedes (although the 'legs' are not true legs, and they</em>
are not related to centipedes), and are always present in all tanks with</em>
live rock or live sand. They are of varying colour, size and disposition,</em>
and a great number of families and even greater number of species are found</em>
in aquaria. It is very common for errant polychaetes to be opportunists -</em>
eating algae, scavenging, or killing small evertebrates as they find it.</em>
Despite their omnivorous habits the vast majority of species are totally</em>
harmless in a reef tank. A very few species may, however, cause problems.</em>
4. The Bad Boys. </em>
Errant bristleworms cause problems in two ways: either by becoming so big</em>
that they can attack things they normally would not be able to harm (ie</em>
fish or aquarists fingers), or by being predators/parasites on valuable</em>
inhabitants in the aquarium.</em>
Bad because of size: Basically a bristleworm larger than, say,</em>
two-three inches can deliver painful bites, and conceivably kill fish,</em>
shrimp etc. Some species also have poisonous bites, and although I've</em>
never heard of anyone dying of bristleworm-bite, there's no doubt they</em>
could seriously inconvenience a sensitive person (normally a bite from</em>
a poisonous species, ie a Glycera, is comparable to the sting of a</em>
wasp). Use caution (and/or tweezers) when dealing with a large worm.</em>
Bad because it's a specialized predator/parasite: Actually very few</em>
bristleworms are parasites, and none on vertebrates, so the fish are</em>
safe (except for very large very hungry predatory worms). Some species do eat corals, and may cause problems. The most known coral-eating species is the Fireworm.</em>
My guess would be 1 of 3 things...accidental, old age, errant/bad boy Bristleworms. Keep in mind I am only guessing and I really do not know for sure.