Oddly enough I was researching last night, barnacles are hit and miss in coral. Some don't have issues but others due and it can cause death in the polyp it lives in. I suspect this is what happened with yours kio. I removed mine by gently grabbing it with needle nose pliers and gently breaking it off. The tissue will eventually grow over the void left by the barnacle. As far as them on rock they would be neat but I personally would choose to remove them. In the oceans they are a nuisance and if left unchecked can multiply rapidly hence ships having to have them routinely removed.