I've kinda of had a small panic attack it looks like my plate coral has some type of worm or feather duster in its tissue. How can I remove it or stop it from irritating the coral?
Assuming it sends out a little clump of feathery appendages that fan out like a rake, then close in like fingers closing into a fist, I'd guess that's a coral-boring barnacle.
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.