It doesn't move because there is food there it is consuming. I'll link you the chunk from LA, as it explains it better than I can:
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During the day it will seek shelter in the rocks; at night it will come out and forage for food, looking particularly for algae lawns and smaller invertebrates like sea squirts or sponges. It can be destructive to corals. It requires an aquarium with multiple hiding places and live rock on which it can graze. The rock formations should be sturdy, since the Urchin could dislodge rocks as it moves around the tank. The Pencil Urchin is very sensitive to high levels of copper-based medications and will not tolerate high levels of nitrates. If water conditions are poor, it will shed its spines.
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