This can be quite common if you have a clown and do not have the proper host anemone with it. Eric's right, he's probably trying to host in it for lack of a better alternative. Clowns typically try and host in Euphyllia Corals if they don't have an Anemone but if you don't have those either than they sometimes choose another coral in the tank such as your Leather. I've had customers tell me that their clowns have killed some of their corals because of the stress they caused them trying to host in them but this is rare. If it continues and does cause stress to the Leather perhaps you might want to consider adding a host Anemone for the Clown.
