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well, it's definitely not going to harm a healthy fish. not sure as far as corals go because i don't know what kind of worm it is. are the appendages on it's sides legs or spines? do you see them moving independently?
then it's a bristle worm and isn't going to hurt anything. if you see large ones here and there, it's because they have an abundant food supply. if you cut back on feeding, you'll not see them as much.