NO, they are not urchins...they are a kind of a pest starfish that eates certain types of coral. Maybe if you have none
it would be OK to let em grow, but they will never be like full size starfish and they are considered relatively bad...
Lemme do some poking around and I will link you some threads. Their pest-worthiness is apparently in the eye of the
reefer. You'll have to decide what you want to do. You can search from here, there is the basic nomenclature in the
thread below.
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http://www.livingreefs.com/pest-starfish-t16171.html