Lets see.
If your goal is to just have more mushrooms.
You could run a razor blade across them on the rock, straight down the middle of the mouth cutting them in to two separate parts.
If your goal is to actually remove them from the rock.
You could scrap them off the rock with a razor blade or heavy duty scissors. Just dig into the rock under the individual mushrooms.
You could cut there heads off

, cut as low as possible leaving some of the stalk behind. The left over stalk should regrow into one or more new mushrooms.
Once removed you can cut the mushrooms into two or four equal pie pieces. If there is still rock attached just glue them to another rock to grow out. If there is no rock rubble attached find a plastic container drill holes in the sides for flow, fill the container half way with some rock and sand rubble, then place the mushroom cuttings in the container and cover the top with cheese cloth to prevent them from escaping. Wait a couple weeks for them to attach to the rock rubble and then glue them to larger rock to grow out. Just make sure there is some flow moving over the container to prevent the mushrooms from rotting.

Not my picture.