Well, @muggle0981 does your overflow box have a removable panel? Marineland overflows have a removable panel.
If so, just slide the panel up and out with the toadstool on it, supporting the toadstool while removing it.
If not, you will need to use a clean, new razor blade to cut it close to the bottom of the base. This will leave tissue behind on the overflow. Any tissue left behind can grow a new toadstool. Any In tank cutting of a leather such as a toadstool will require a large water change and use of granular activated carbon in a reactor or a bag. when you cut them they will release toxins into the water that will irritate or possibly kill other coral.
Once the toadstool is been removed, rinse it thoroughly with clean saltwater at temperature. It can be reattached to rock using rubber bands loosely holding it in place.
I just recently removed one of my very large toadstools from my display tank to frag.
I have removed several by freezing some RODI and holding the ice against the base. Pull slowly and use something to pry the edges. I wouldn't cut a coral unless you can dip for infection. If its not possible then a razor blade at the base works