Wow, that is a small urchin! Just a baby, probably the "rock boring" urchin Echinometra a very common hitch hiker. Hard to know for sure though. This will get larger, and as with all urchins in time may be a bulldozer of poorly attached frags and such. All urchins, IMO, have some risk of eating soft corals - zoanthids or encrusting things in particular, but this is a limited risk, IMO. I don't see a particular reason to worryl.