Early morning before the Mushroom have opened up, with a screw driver try to brake the lr around the mushroom. Grab the lr with mushrooms, take the screw diver and press it next to were the shroom is attached, apply pressure with the screw driver blade tip. Live rock should break , flake or crumble off with the shroom on it. With the small amount of lr still attached to the mushroom, makes it easier to glue to lr.
Smaller size screw driver tend to dig in lr better
If you can pull live rock out of tank, do so.
Get a dremel rotory tool with diamond bit cut wheel, make one cut , wedge screw driver into cut, dislodges mushroom with bigger chunk of lr