This little guys is a super fast swimmer and is segmented.
I haven’t been able to find anything on it was swimming around my tank at night and dropped to the sand bed as soon as I flashed my phone light on him
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I have a few worms that have a similar behavior, notably the dorvilleidae worms and whatever that very spiky refeugium/Acanthophyllia worm was. (You know what I'm talking about)Probably a bristleworm (polychaete) epitoke (the reproductive life stage of a bristleworm), but a few kinds of bristleworms can swim regularly as well.
The pic's too blurry for me to narrow it down beyond that - sorry.