I have one and it's one of my favorite critters.
That said, they will collect things on their backs. Thus, you have to have all your frags solidly mounted.
On the back of mine, I have found, mushrooms, snails, and the best one was the floating thermometer!
They eat coralline algae off the rocks and glass; they also eat green algae. You can tell where they've been on the rocks because they will eat the coralline right down to the rock. Don't worry, the coralline will grow back.
Personally, I like the color of the rocks with an urchin in the tank. Solid purple is unnatural and rather boring.
Best of luck,
Roy