i believe some majority of all spiecies of leather goes thru stages of stuff like that in your pic either when introduced to a new tank from the store or the i guess you can call hibernation stage, where the
leather coral seems to be crashing looking ugly with a slime over, then when that stage is over, it blooms out again, as long as you don;t see at the bottom of the stalk where you see tissue breaking down, like holes. i have had many different types of leathers that have that type of slime or colorization but only to come back even most beautiful, correct if i might be off.