They're usually not a problem, but they do generally cause a little, dead/deformed patch around where they're burrowed in (and that patch tends to grow as the barnacle does). It being that close to the mouth is a bit concerning to me, but I haven't seen it on enough LPS like this to know if it's an issue.
If you want to get rid of it, I'd imagine you would need to either crush it in its burrow or carefully inject something into it that would kill it - either way, you'd really want to be careful with it being in the coral.
