Why would you want to remove it from your tank? How big is it. If its under 5-7 inches long (one arm all the way to the other) then its a sponge brittle star. They aren't harmful and will only stay in one spot (usually is the holes of a rock or a coral) They are good guys, They clean corals if they are hosting one, or just sit there with there arms out, and its almost impossible to remove them because no matter what you feed them, they don't leave their hideout and only get food thats placed right on top of them. Believe me I've tried to remove my first one that was a hitchhiker, thinking it was a bristle worm, I tried for 2 days, and I just ripped its arms off.
They are really common hitchhikers, you probably don't realize it, but there are probably many more inside your rocks. I ripped a piece of rock from a bigger chunk, there were 4 three inch long ones under it.
I don't see why you'd want to remove it. But if you really want to, I'd say its almost impossible, you could pinch all their arms off with tweezers until they die. Kind of harsh and cruel, but thats the only way I'd say.