Mine was a hitchiker that came on the only batch of rocks I got, so he was in there for about 6months, I knew about him for 4months before I did anything. As long as your hammer coral isn't on the sand bed it should be kinda safe. I've never heard of a fireworm swimming, and mine would only come in and out of holes in the bottom or rocks, the highest i ever saw him was about 1" up comming out of a hole and going into a lower one, I just tryed to keep mine off the ground, but my anemone would do what it wanted and my star fish wasn't lucky either, every thing else was fine... it is rare to have one, i do recall thats why dr. ron didn't think I had one because its not a regular occurance, but you could try to get it out before it gets big, they make small tubed with spikes directed into the tube, you put something meaty in the tube so the worm goes for it and can't get out. The tube is only 5-6" tho which was a third of the size of mine, so i didn't buy it.