Definitely a little rock boring urchin. An uncommon but not unheard of hitchhiker. It's probably been in your rocks the whole time without you noticing; they chew little tunnels through the rock to hide in and come out at night. They're not strictly reef safe as they can destabilize your rock structure, though one that size won't do much. If you see it again you should grab it if you can, though I will warn that they have a very strong grip on the rocks.