slide the blades of a clean, oil free pair of scissors under a few selected mushroom heads and snip 'em off.
Catch the heads as they float off and then hold them in your hand (underwater of course..preferably with latex gloves on)
Now take your scissors and cut each shroom into quarters, making sure to cut across the center (do this because a piece of the center needs to be included in each piece)
Now you can either let them float away onto the substrate where they can grow, or take some bridal veil or fine netting and secure the new little pieces to a rock of your choice.
The stump of the shrooms
you cut off will grow back......
Plus each one of the quarters will grow into a new shroom.