They aren't really bad. They just eat leftover food and etc. many people leave them in the tank. I personally hate them because they creep me out, so I remove them.
It might not be a bad idea to check your rock for any other critters. They sell products to soak your rock and kill unwanted hitchhikers. It is not doing anything bad to the corals or live rock. I use it as well when I buy new corals. Good luck
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