I've had one in my small (37 gal) reef for several months now. Mine's not at all interested in Valonia but quickly disposed of all the
turf algae in the tank and now is working on the Caulerpa. I've never seen any evidence of it eating or bothering corals--but it does like to pick at algae growing among Clavularia and Zoanthids, which sometimes causes them to momentarily contract. No damage, though. The critters are omnivorous and very greedy eaters--at least mine is. It will gobble up anything I throw to the other fish (live brine, frozen worms, even flakes). This can make it a little difficult to feed small timid fish. My foxface is not at all aggressive, and really a nice addition, but I'm reconciled to the fact that he might outgrow my little tank.
Best,
BA