I think the safest bet would be to remove the rock its on and hang it upside down just above the surface of the water so the nem releases naturally into your hand or critter container, whatever you want to grab it with.
That or you can seal some ice cubes in a ziploc bag with no air in it and massage the foot gently- the cold it wont like and hopefully release as well. But I wouldnt try to pull it off the rock its on, you may tear the foot- no good will come of that.