If you are positive salt creep landed on the coral, that certainly what it could be from. Undissolved salt is very damaging to marine life if they come in direct contact, the salt will quickly burn whatever it touches. Leathers are incredibly hardly corals, and it should come back in time.
However, toadstool corals are known to shed from time to time. If you recently done something to the tank, done a larger then normal water change, added/removed a powerhead, this could also set the toadstool off into shedding mode, as the coral begins to get rid of it's outer 'skin' (not really) layer.