What the heck do I have in my tank?

[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.