My urchin did the same thing, it would move for a little bit but then it would stay in one spot for a couple days. But afterwards it would move again. Your pencil urchin should be fine, it probably just likes the spot it's in. When an urchin dies it usually let's go of whatever it's holding on to and just falls off. Also, cuc will start to move towards it. Also, do NOT pick the pencil urchin up with bare hands. It'll sting the hell out of you. Inside each stem has poisonous stems that can inject and eject. They'll sting you if threatened, so if your gonna pick it up use gloves.
Other than that give it some time. Good luck