Leathers are funny like that. Mine shut down sometimes for a week. I noticed when I was doing waterchanges every week they were open. After biopellets I do it every three weeks and now find that they stay closed for the last. There is prob an element like iodine that must get used up that triggers the behavior.
Leather corals simply shed. It's natural and to be expected. As already mentioned, providing a higher flow in that area or moving it to an area of higher flow will allow the currents to carry away the shedding. They (leathers) will shut down, shrink, shed, and afterwards in a time of its own choosing, extend its polyps out and be as beautiful as ever. An indication of low flow from observance of the image is the substrate in its immediate area.