yup, by your description, that is definately a barnicle. It's a first for me on a plate coral. Usually, they happen on euphilia corals such as galaxia ... provides for better protection.
so... its good it won't spread? yeah i've seen a banarcle(on on of my hermits) they're pretty cool... but not on my 30.00 orange plate
but it won't bother it and stuff? By the way look bac at the pic... is that "horn" on my plate from a past skeletal crack? a barnacle and a horn SWF.com sure has some wierd plate corals