Depends what they are attached to. If on the sand the foot is likely attached to the bottom glass and its pretty easy to gently peel them off using an old credit card (or other thin plastic object). If on a rock then I'd probably just save the time and chisel them off with a flat head screw driver and hammer. Just be careful to position the screw driver head far enough behind the foot as to not damage it. If you are concerned about damaging the foot with the chisel method, you could try changing up your flow/directing a power head at them to see they are start moving on their own. Once they have loosened the foot you could then try peeling them off the rock.
