When you see them on the glass you can remove them by hand, I have used tweezers to get them off my rocks. I only saw a few in my tank about a month after setting it up. They multiply rapidly and you won't know if they are the coral eating variety until It's too late. I played it safe and removed mine.
So I purchased an urchin yesterday. They are so cool I couldn't resist, didn't make it down to the lfs I was planning to instead I got a stay in the ER for 8 hours. Oh well I will get another chance soon