Well it could be all kinds of things. With out a photo its very hard to get it right. Possibly tube worms, baby chiton, barnicles all kinds of stuff comes in on live rock. Questions that may help others narrow it down. Where is your live rock from I.E. Tonga, Figi, Haitian, Florida, or where ever. How much light on your tank. This will sound wierd but what are your livestock, and inverts? What are your tank parameters. Normally when you have a overgrowth of filter feeder, which I think you are describing, it means the tank has got more nutrients than it needs. How much and what kind of light. What is the temp on your tank? All these sound unrelated but they do matter. A picture would be a great help as well. If its the little white shelled tube worm that is all curled in a ball with red feathers they come when nutrients are too high. If that is what it is, a better idea of what is in your tank and where the parameters are at will help others to help you beat this. Steve