What the heck do I have in my tank?

[untamed]

Until you get a better look at it, guessing about identity is a challenge.

One thing strikes me though....I don't find that most bristle worms move very quickly. They kind of crawl along pretty slowly. Eunicid worms, however, retract back into their tunnels like lightning! So you might have one of those.

If it is, some would say that they are problematic. (Good luck catching it!) I have many of them in my small tank and haven't found that they do any damage so far. Some can achieve great size, but others never get very large.

[sumpfinfishe]

Maybe it's a bristle starfish!

[seashells]

Would be tough to know without a picture. If you see hairy legs and pinchers then there are several crabs that would fit. Does it have red eyes? Do a search for live rock hitchhikers and crabs and see if you can find anything that resembles your alien.

Doug

[Quagmire]

The hairy legs part makes me think of an Emerald crab.Maybe try a search for Mithrax.See if you recognize if from a picture.