There's several ways to do this. An easy way, IF you can get to the rock it's attached to, is to gently break off a piece of rock that it's attached to. If that doesn't work, some methods I've used that worked for me are:
My best results have come from rotating the rock that it's attached to and aiming a powerhead at it. This seems to make them want to move. When they start moving, you can usually GENTLY, with your fingernail, credit card, butter knife, start at the part of the foot that's a little raised as they move and GENTLY tease the rest of the foot up off the rock.
Another method that I've had mixed results with is to hold ice cubes against the foot. This hasn't worked all that great for me.
I actually had to remove one today and was able to rotate the rock, putting the RBTA right at the top. I then used a screw driver and placed it on the rock, near the foot and tapped it a couple of times with a hammer. The rock cracked and I was able to lift off the broken portion, along with the RBTA.
I've also had NONE of these methods work and had to move corals instead...lol.
Good Luck!!!