As I was saying, corals like leather or even just mushroom corals, they are of some the easiest corals to keep and brain corals as well are easy to maintain for these corals can be kept under VHO lighting.
Sorry, I need to hear what is your calcium level is and if you ever dosed with Iodine? Also do you dose your sump (RO water) now and then with Calcium Hydroxide?
These were some the very first corals that I even started with, I cannot understand why you be having any problems at all for the leather corals for one thing need time to adjust and if you been handling the coral often after you placed it into your tank, this would had placed much stress on it. You need to place the corals and try not to place your hands into the tank if you can help it.
On lights, I'm no expert but I do know the corals that you are mostly keeping can be kept rather well even under VHO lighting. It could be also that you could done to much water changes but the same thing on that, one needs to know of more details.
So still, people would need to know what are your tanks parameters for there has to be some answer to why this is failing, your leather coral that is
You do understand that when you first added this coral to your tank that it was already under much stress? So you need to know where you would place the coral, get it in and leave it alone.
Let me ask you something, did you know that you upset your tanks perfect balance when you keep putting your hands into the tank? I not even in eel tanks put my hands inside them unless I really have too
For as for remove this or not, if this coral isn't falling apart at the lower part of its body, then cut off where it is dead only and cut this with a very sharp cutting tool and also in one clean cut. Don't cut as your cutting wood or so, the cut must be in one motion and be careful not to tear. But if it is falling apart and not much of a solid body to it, then remove it now, Sorry.