If you are going to remove them from a plug, you are best to do it well before they grow onto it. If they have grown onto the plug (actually bonded or grown skeletal mass on the plug) you can not without fragging it.
I like to remove mine from the plug whenever possible as it looks more natural I believe. I use a sharp knife or rasorblade and find a spot that i can slip it between the plug and frag, then twist or turn the plug along the blade to loosen the bond.
!!!!!!be careful!!!!! 1. sharp blade. obvious reason. 2. If the frag is thin, meaning the skeleton is thin, you can snap the skeleton without removing it from the frag, also, if the frag endured a decent amount of pressure when being fragged, micro-fractures may exist and will break before the frag removes from the plug.
IF you snap a frag you have two options....
a. cut the flesh with a Sharp and sterile exacto knife\ scalpel.
b. place the frag which is hanging together by skin in a hole in the rock and more often than not it will heal, but you cant move it easier from then on, so be sure that's where you want it. I have done this numerous times, and i have not lost an LPS frag yet. I have even stuck the 2 pieces (still attached by skin) on a rock with each halve pointing two directions and the acan lived, and flourished. I have done this with favia as well.
---do not glue the halves back together. the glue or epoxy may severely irritate the exposed flesh, which is no good. hold them with a rubberband if you really have to do something about it for peace of mind...(braces rubberbands work great, and they are easy to get)