that is the beginning of the flower pot dieing once it starts it's only a matter of time before it spreads , most flower pots last about 6 months some get lucky they last a little longer, they just are a tough coral to keep in a closed system
Did you target feed it or just broadcast it over the entire tank? I would shut all the flow off and try spot feeding it with very small pieces of food. It may help, though these are better left in the ocean, as stated. there are some tank raised specimens but they are few and far between.