They are scavengers and usually not a threat to your fish. There are some that are capable of stinging though. I pulled what I think was a Bearded Fireworm out of my Anglers tank a few days ago It was about 10" long stretched out, 7" and FAT when tensed up. Ugly little monster.
As for good sign/bad sign... it's a good sign. Your tank is "live". If you add any good live rock pieces to your tank you are going to see them eventually. If you start to see a ton of tiny ones though, you may want to find something compatible with your tank that eats them (most Wrasse do) or try a "Bristleworm Trap". Also, a large population is usually due to over-feeding, so that's a good way to monitor feeding your tank.
You should Google images of Fireworms and Bristleworms.