Charlie's right, sounds like Bristleworms. These guys do a good job on clean-up of detritis, dead/decaying animals and left over fish/coral food. Their population will rise and fall with their food source, once it reaches a certain point. Your tank is pretty new so they probably still have a good food supply in the die-off and shedding detritis from your live rock. Once your tank gets more established though, any further large increase in bworm population could be from over feeding your tank.
I like Bworms as part of my clean up crew. Just don't pet them their spines have an irritating sting.
I have them in my tanks so thay could have hitchhiked in with anything that you got from me. Probably on the rubble that the feather calurpa was attached to. Are they red on one end and browner on the rest of the body? If so I agree - bristle worms.
They swim all over. There is more than I can count on my hands. They look like fish. Just really small. The catiplillar thingys are going down in numbers. But I will check again tonight. I might just turn the lights on and take some quick pictures. But yea all have a good one