Some times what looks like a plump bit of skin on the bottom of a zoa in a few days/weeks turns out to be a new polyp!
I've had some of my zoa frags sprout 1 polyp every 2-3 weeks.
Also, if you have a regular aragonite or somesuch frag plug, you can just...'pop' off the bottom of the plug.
What I do:
Put the frag on the plug between my pointer finger and thumb (where the pad of my thumb would touch the 'middle' knuckle of my pointer finger IF the frag disc were not there)
Place the pad of my thumb on the side of the frag plug stalk.
Hold the disc portion of my frag disc with my 'free' hand.
Increase the pressure of my thumb on the stalk only.
The stalk snaps free and then the disc with the frag on it is safely held by my other hand instead of being flipped across the room.
Done.
Every time I have done it, the frag has been fine, the disc didn't break, and the stalk of the frag plug came off cleanly.
Don't try if you are scared. I would try on an unused frag plug first.
Also, I've never done this on a coral that has encrusted very far down the stalk of the plug.