Leather corals slime a great deal and are almost impossible to glue to a rock or plug... but it can be done.
First, a loose rubber band to hold it in place for a couple of weeks can work. Or covering it with very fine net, mesh or bridal veil to hold it to the rock for a couple of weeks. A 3rd technique is to take a long needle and some thread and go through the leather and around the rock or plug and tie it off as a means to hold it down. Most leathers will heal and attach to the rock on their own in a couple of weeks if held in place long enough.
Now, I've glued some leathers to rocks or plugs and had some success. First take the foot of the leather and hold it up. If it's fresh cut, squeeze it a bit to get some fluid out of the leather. Next, while still holding the cut end up, dry the foot or raw end with a paper towel. Now put a layer of super glue over the entire foot or raw end and hold it up while it sets up. A fresh raw cut may need 2 coats of super glue. Now you have a good dry base (like a scab) on the coral. Coat it again and set the plug or rock on top of the foot and hold it there gently. Do not squeeze the coral or you will force fluid to flow and disrupt the super glue 'scab' that you made.
I used this method on some 1" thick trunks of branching leather corals. The fresh cut end is like a bundle of tiny straws that are all full of fluid. A gentle squeeze will get lots of fluid to flow out the fresh cut end and when you release the pressure the remaining fluid will go back down inside the tubes and leave open ends. I dry the ends with a paper towel while still holding the leather with the cut end up. This technique works for me about 80% of the time. Otherwise I use the needle and thread technique.
