Has the polyp bailed out completely from the skeleton or edges are slowly receding? The first is known as "LPS Polyp Bail out" and the second is commonly called "Slow tissue necrosis" (STN)
Elegances are notorious for polyp bail out and there is nothing you can do, AFAIK. The polyp will die, so it is better to remove it before it starts rotting.
There is some literature about the behaviour, some researches believe this is the last resource in an attempt to find a better place. I am not aware of any evidence of it being able to survive either in nature or in captivity.
In addition, Elegance is considered by many as a difficult coral to keep and some are of the opinion they shouldn't be sold, due to the high mortality rates. This is specially true in countries where they have to be imported, due to stress caused by shipping. Here in Australia, it seems many more people have success with it because shipping times are much shorter.
Also, the forums are full of reports of thriving elegances that "bail out" for no reason.
Therefore, it is recommended that beginners should avoid Elegance coral.
I would check with water quality is pristine and if the coral isn't receiving direct flow.