Sounds like a mantis shrimp to me
, although it could be shrimp or crab. I would suggest building a trap or buying one from your LFS to trap whatever it is, as your just starting the tank it will be much easier to catch whatever it is now rather than later. If you don't have any fish in the system I wouldn't add any at least until you identify what it is. The last thing you want is a hungry mantis and a new clown or goby in the same space.
You could also search the DIY forum here and on RC, where you will find great threads on how to build traps, simply search the word "trap".
Goodluck and if you can, try to get a snapshot even if you have to use the flash on your camera.
Sorry didn't see the word
"starfish" in the last sentence, maybe you have nothing to worry about after all, first trap the sucker or at least get a picture before adding anything else to the tank tho
