Looks like there is algae in the bristles?… Or is that just the way the pic came out? From your new information, I would say this is a very small piece of the worm. (I don’t read directions well)… I just read your entire first post. Sorry I usually scan post and jump right to the pics… (It was my NYE resolution to get better at this

) So sounds like you decapitated / debutted

it… I would have to see a larger specimen but looks kinda like the tip of the only fire worm I’ve personally experienced. My first tank BTW…
Might not be. Check out this small reference to see if it might help.
The identification of Worms found within live rock and live sand
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