Leather corals will suddenly turn dark with no polyps for no reason. They do this and they even look like they are peeling. Some say this is waste removal action by the coral. Not much can be done except maybe add a little flow near the coral.
I've had some do it for a week or two, then suddenly come back to normal.
Leathers randomly close up like this all the time. Usually a change in flow can trigger this, but it's also a tactic to prevent algae growing on the tissue. If it looks waxy and shiny that's also normal. Give it a few days to a week as suggested above and it'll likely be fine.