If the pedal disc is in a crevice or hole, you'll have to bust the rock. If it is attached to a flat surface, you can use a credit card or rounded nail file to gently pry it off. If they are too hard to get off the rocks, then it's best to leave them alone until they start to move around. Do not hold them out of the water upside down, do not point a powerhead that them, but the ice cube trick is safe to try.