Depending on the coral really. Some will overgrow the frag plug and attach or encrust themselves onto the rock. This is even easier if you drill a hole in the rock for the plug to fit into.
I have only ever glued 3 frags, and they were of rics. When I got them, they were on a miniscule amount of rock...talking smaller than the size of the fingernail on your pinkey. I just got a larger piece of rubble from my fuge and attached them. The first time I did it I used a moldable 2 part epoxy. It worked ok...but it was a pain and because of it being a softie was difficult to push it into the epoxy and took 2 attempts to get it to hold. I just got done using actual glue on my 2 new rics. So much simpler. Just threw some glue onto a frag plug from an SPS
I killed, stuck it in, and held for 30 seconds. If it was still slightly loose, I added more glue around it. Depending on the glue, this is how mine was, adding a small amount of saltwater makes the hardening process go faster.