Something strange in my aquarium!?

I have a 3 month old saltwater aquarium, with a few polyps, a couple of fish and a hand ful of snails and hermit . I looked in my aquarium today and to my surprise there are small greenish-brown tubes about 3mm talk with 12-15 feather like extensions coming out the top! They're in my gravel and on some of the live rocks! I was told to expect random creatures in my tank, so I'm delighted to see my aquarium maturing, but what are these?