I can't see any mucus in either picture. Leather corals will shed a mucus layer to rid themselves of algae or dust. That will also periodically sulk like an anemone where they shrink down only to reinflate for no apparent reason. Really those two pics don't look bad. The polyps are only partially extended, but if it is sliming they would likely not be out.
Check you water parameters, particularly nitrate, calcium and carbonate hardness. Also look for any other tank occupants that may be harassing the leather, though I don't see any other corals in the pictures. It could be a hermit crab or fish that is causing the distress also.