Why do you want to move it? They wander to the area that they prefer. As long as you want it in the tank, just leave it alone. If you're trying to remove it from the tank entirely then sometimes putting some ice in a ziploc bag against their foot will help get them to loosen up. Sometimes pointing a PH at them will annoy them and they'll wander away from the direct current. If you can get your fingernail in between the foot and the rock you might be able to very slowly peel it off, but it's very easy to injure it doing this, so you have to be careful. If you're not trying to get it out of the tank, you're best bet is to leave it alone.
B