First just a few general things when acclimating leathers.
1. like all coral provide it with a stable mature environment.
2. place it in a secure location so that the coral may move in the current but not the piece it is attached to.
3. medium to medium strong indirect current seems to work best for me. If detritus is allowed to settle on the cap of the leather it will retract its polyps and shed in an attempt to clean itself.
4.while maintaining the correct "levels" in your tank and making appropriate adjustments is ok...a regular husbandry routine that includes water changes will provide a more than adequate environment for raising most leathers.
5. Once you have placed the coral LEAVE IT ALONE h* leathers can take from 1 hour to 1 week before opening. Every time you move it from one "suitable" position to another you are simply prolonging the acclimation IME/O.
6. Add supplements with caution as the leather coral (and many others) can act as a signal that something is wrong in the tank only after it has looked healthy. Making adjustments to water chemistry based on only the leather not opening can lead to creating problem that may not have existed otherwise. No offense meant
As for your leather...given the series of pics I would say it is in decline. If it were mine I would try the following.
If you haven't done a recent water change I would...25%
If you are not running a high quality carbon I would...remember some carbons can leech phosphates.
I would remove the leather and GENTLY with a very soft toothbrush or terrycloth clean all signs of algae from the cap. In several of the pics I notice what appears to be red algae on the edge and top of the cap. This need to be removed or the area under the algae will begin to rot. After rinsing the leather replace it in a medium flow section of your tank so that the base is secure then LEAVE IT ALONE
You can then concentrate on the water chemistry and see where it leads but at this point you have done everything physically to the leather that you can (short of fragging...I wouldn't yet)
Good Luck
SiF
PS Thomas712 I have started a new thread with questions for you. If you have a minute please stop by