I've had urchins loose spines and return back to normal. More than likely your urchins starved, either not finding adequate food in the tank or previously starved from the supplier or a combination of both. I'm not sure the film type algae that grows on your glass is enough to sustain an urchin and unless algae is plentiful you are limited to how many you can keep in a certain size tank.
What size tank? Exactly how many urchins? How old is the tank?
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