Hah, thanks for the input guys!
My tank had some caulerpa when it was initially setup, but I removed them all by hand long before this one was introduced into the tank, so I have no idea whether mine eats or not and I only see them at night...but I supposed all urchins are beneficial one way or another
only got worried cuz it grew so much so fast.
I had to draw it out with measurement cuz I couldn't remove it from the tank =D and there was nothing to compare to anyway =P