I would just put the glue on the coral and then apply it on to the rock. If you do it this way, you want the glue to "skimm' over and then you have the time to put it where you want - of course you gotta have a place in mind and work fast - about 5-10 secs underwater. I do this all the time. Too hard to move LR around for me.
If it's a bigger piece and super glue won't hold, I use Aqua-mend, two part epoxy for spas and ??, just break off a piece, knead till the color is uniform and (this trick is great), apply a little blob of super glue on top of the ball press it on to the bottom of the coral and then do the same for the other side, then place on your rock. Epoxy itself will not hold anything but the super glue gives you some time for it to set up and then you'll have to use a screwdriver to pry it off. You can get it at my favorite home away from home - HD and it's only $3.00 a tube. Will last you for a little while, unless you have dozens of coral to glue down.
hth
As mentioned, , you can move them after you glue them down but sometimes it'll be a little hard to, just becareful when prying them up.