It depends on what kind of urchin you've got. I've got a pincushion that has proven to be a model citizen. Sometimes they're white, so that could be what you've got.
Why don't you post a picture so someone can id what yours is.
You'll get mixed views on urchins in general. Some swear by them. Pincushions and tuxedo urchins seem to do a good job eating algae and don't seem to eat corals. However, pincushions will knock things over and pick up small frags and carry them around the tank. Long-spine diademmas are thought to be reef safe, but they get really big really fast in good conditions.