[Sk8r]

Looks like a eunicid to me. And thank you, Sushi Girl and Wet Shepherd, for those pictures! Good work!

Understand, there are very, very, very few of these things (the eunicid, the caulerpa algae, and the crabs) that are bad. The rest are all part of a healthy reef tank, and if you don't have them, then there are places to get them. There are dealers that will sell you these critters. Don't be too eager for coralline algae---especially if you have an acrylic tank. The stuff is pernicious. I spend more time scraping it off than I do green film algae.

This proliferation of small creepy-crawlies is why I prefer my live rock un-fussed-with. I don't want to kill off its diversity---a diversity which goes right down to the things you can't see, and which means that you've got a healthy community of things processing any waste or problem. Given really good water (check my sig line for params that work well) they'll keep themselves in balance. Copepods prey on pest algae. Other things prey on copepods. It's that circle of life thing. Cue music.