what i've done in the past is:
grab a brand new razor blade
use the NOT SHARP side to slowly and gently peel back the edges of the foot
for some reason the razor blade metal seems to "stun" the foot and it lets go
don't be alarmed if it doesn't immediately attach back to another rock when you put it in...this method seems to stun the foot for a few minutes...
i've used it a buncha times when i was moving my tank...the nems kept landing on top of acro so i had to move them a few times, till they found happy places where they weren't stinging everything