Mystery Solved!

Sabellariid, Lygdamis spp.

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Can anyone ID this thing for me?

Each tentacle is a little less than a cm in length, and the entire creature lives inside of that sand-tube. It will retract on contact.

I've never seen anything like this before and haven't been able to ID him. He's been in my tank for about a year and hasn't spread, this is just the first time I've been able to photograph him. He was originally attached to the rock base of one of my SPS, but he moved after I buried him in the sand.