It just doesnt add up. You have rock from a different part of the world than "fireworms" live, so that rules them out. Then bristleworms dont eat mushrooms, so that's not your culprit. Thats just not what they eat or how they behave. They are scavengers not predators.
Wait, where did he say what part of the world the rock came from. This did happen to one of the members of my club. Had a fireworm hitchhike in on some coral and it started selectively eating his mushrooms until he caught it in the act. He actually trapped it with mushrooms in the trap
Well that's an interesting point, not hitchhiking on the rock, but on the coral. Still, do you see them out in the daytime? Are they a deep, fiery red or just dull pink/red?