It's not always easy, but there are a couple options. If you can get to the bottom of the foot, often a razor blade to cut it from the rock will work. You have to be patient and secure the mushroom somehow to the next rock (dab of superglue sometimes works, rubberbands are another option). Otherwise, sometimes I'll chip away the rock below the foot of the mushroom and then just glue that piece of rock to the other rock (again, here I'd use superglue gel).