Coral? or some sort of tubeworm?
I have these very small, dark red or purple tube creatures living on almost anything in the tank. They probably can from the few pieces of live rock that I have. They are made of some hard material. The tube is curled at the base where it sticks to the rock. A wormlike thing lives inside. When they die the tube they leave behind becomes white. The creature itself is pinkish and it will at times stick out a few tiny white tentacles out of its tube. When I stir up sediment or dirt or whatever is in the tank, it will send out one of these really long strings. The string is white and barely visible. Any stuff floating in the water will stick to this thread and eventually the creature will pull this thread into eats tube to eat whatever it caught. What are these creatures and are they good or bad?