ID on urchin

[Victoria]

I found this urchin in my tank about three months ago. I have watch it grow to the size it is now. I have never put one in this tank. I did transfer some LR from another tank that had an urchin in it. When I first spotted it , it was the size of an eraser on a pencil. Is it going to cause me any trouble? At the present time it stays in the back. I rarely see it out and about.
Victoria

 

[Victoria]

Here's a close up

 

[fijireef]

Echinometra viridis, the Reef Urchin. 2-3 in. diameter. Pointed purple to brownish spines with a white ring around their base. Tropical West Atlantic. Feed during the night (hide in coral, rock by day) on algae.

 

[The steven liu]

Some people call it purple urching, some call it rock boring urchin. I have a couple in my tank. Very cool animals, they are reef safe (most of the time) but can knock stuff over if they get clumsy. They will eat all kinds of algae including hair and coraline algae.