Issue with leather coral

[halfdoz]

I have had this coral for almost two years. It has grown and done very well but about 2-3 weeks ago this white spot showed up and it’s slowing growing. I think I’m going to take it out and dip it but does anyone know what this is? Everything else in the tank is fine.
This morning
Nitrates 5
Alkalinity 8.4
Temp 79.0

 

[reef lover]

Looks like hes dropping an arm,self fragging,they do that occassionally.