I would get rid of them, or a better solution (for the crabs anyways) would be to put them in your sump (if you have one). It certainly doesn't hurt to get rid of them, especially if you are not sure if they will be a pest.
One trick to catching them is just to put a glass leaning against the rock where the crabs hide, and put some food to attract them in the bottom of the glass. Crab goes in to get the food, but the glass is too slippery to get out. Sometimes it works, sometimes it doesn't.
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