Sometimes if you point a powerhead at them they will dislodge. They usually don't like really high currents and will try to move out of the way. Make the stream enough so that the head of the mushroom is actually getting pushed up an out a bit, the foot will usually start letting go in a day or two in an attempt to move...you can nudge the foot with your finger at this point and it should drop off.
The other method, would be to use a dull blunt instrument, like a screwdriver (flathead) as a prying tool. Try to pry a bit into the rock, not touching the mushroom, and dig it off the rock. This doesn't damage the mushoom and you can frag to your desire.
Mushrooms are funny: Seems like either people have too many, or, are looking to get more... personally...I'd try to frag em' and do a trade, or make a few bucks, you have nothing to lose...
cheers,
B.