Looks like a standard bristleworm to me too. If you look at your tank at night with a torch, well after lights out, you'll be suprised just how many you'll see.
Bristleworms are detritus feeders and they can't eat or kill a fish. They have no poison. The hair like spikes are just to stop fish from eating them, but some species of fish will eat them. If you saw a bristleworm on a dead fish it got there after it died and is just cleaning up so to speak.
Just for anyone who is interested here is an old pic of my Falco Hawkfish eating a large bristleworm. He got spiked but was totally fine, the bristleworm however didn't stand a chance
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