Most Open Brain Corals I've seen - when first shipped or purchased - are normally not in the best shape.
When you can see the tissue receding, and the skeleton showing - they need some help from us.
Mine does A LOT better on the sandbed than up in the rockwork.
They should be placed on the sand.
Lighting is important - but these guys NEED TO EAT MEAT.
Several times a week is what they like - IMO.
Thawed out frozen or fresh shrimp, krill and squid, placed right on it mouth - is what I've found works best for mine.
If it keeps losing tissue, and looking bad, it may have a bacterial infection - and may die.
You can pick it up and hold it just to the water surface, keeping it wet but exposing the questionable part to the air ........ and take a whiff. It it reeks like dead animal - it may be infected.
If not - you're in good shape.
I've heard some folks do an iodine dip. I've never done this so I don't recommend it. Maybe someone else here can tough on this procedure.
Get some food into this guy and see what happens.
If you see it's feeder tentacles around it's edge - it's hungry.
If it's all curled up in a tight ball - it may take some time for it to respond to feeding. Squirt a little frozen food "mush" on it with a baster ( pumps/powerheads OFF ) and this may entice it to eat.