Spirorbid worms. Harmless and will go away eventually in the DT, maybe not so much in the fuge.
"These animals brood their larvae in either the operculum itself, or within the tube until ready to metamorphose, and then release very short-lived planktonic larvae. These larvae often settle within minutes (and almost always less than an hour) of their release, and can therefore spread rapidly within a reef aquarium. This generally happens early in the life of the aquarium, and in a well-maintained, clean aquarium, the initial population explosion of these small worms quickly slows and they are almost never a problem in any aquarium. If the population continues to climb to the point that all surfaces in the tank become encrusted with these worms, it likely indicates a high suspended organic problem, and you should be more concerned with your water quality than the presence of these polychaetes, per se ."