too much of a lack of info for anyone to give you any meaningful advice.
watch it close, listen for clicking.
Bristle worms and mantis shrimp and pistol shrimp all look very different, but based on your description, it could be any of them.
best to be sure before you consider any advice or diagnosis.
wait until your lights are out, and put some food near where you saw it retreat (not with your fingers) and see if you can't get a better description.
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