Have seen them almost as big as a diner plate. From the looks of it you touch it and it retreats into rock it's aptasia. Get it to retreat then glue it's opening shut
Looks like you can remove the rock from the tank. I'd recommend doing that before trying to mess with it as to prevent an outbreak. Mess with one, end up with three.
Yeah, if you try to cook it with hot water or a laser, or pic it apart you could end up with more. If you glue it shut it pretty much starved to death and doesn’t spread. Although with one that big would guess there’s already others