mine does very well out in the open. i feed it in a tub once every 2 days.
That would be temperature related in my opinion, 31°C is very warm for a tank.
I also normally don't recommend feeding these types of corals daily. They can over eat which allows food to rot in their gut faster than they can process it, which causes tissue recession. With that said I still suspect temperature...
Alkalinity at 7dKH should be fine, assuming it's stable.