Please help me ID this

[alex308]

Hello! New to this whole thing and saw this in my tank. Not sure what it is. Please help me! I recently lost a sea urchin and am trying to find the cause. Not sure what this is or if it’s an issue

 

[OrionN]

I think what you are asking is the worm in the middle of your photo. It is one of the bristle worms, part of the fauna and cleanup crew. Not a problem. It is not what kill your Sea Urchin. That is what we want when we want live rock. In the 1980's, I beg the LFS owner for some of these worms to put in my marine fish tank. They did very well with my under gravel filter and eat all the food in the gravel bed. Multiply like crazy and feed my fish with spawn.