If it is loosing flesh the outside edge, wound will get an infection and sometimes the bad bacteria can out compete the good. If you superglue the flesh to the skeleton and close up the open wound it can help. Also in my experience dosing good strands of bacteria can help get the good stuff back on top.
Keep out a close eye, tangs will nip at the algae and can irritate the coral and peppermint will make declining coral die much faster as they scavenge the death.
Last thing is too make sure a more aggressive coral has grown too close. I had a huge space invader and Duncan colonies very close together, they both got bigger than a softball and one day woke up too the space invader destroyed by the Duncan. Up until that point they had never had a problem, the Duncan put off thin threads that killed the Space Invader. Lucky I had a small frag going in another tank.
I keep my chalice coral between 60 and 100 par, but I do not think the lighting is your issue, unless you have made a drastic change in your lighting.
Corals sometimes just go, no explanation, I loose them too so frag and share so you can get it back if you loose it.
Do you carbon dose or run biopellets? If yes maybe discontinue for a bit. They can feed bad bacteria as well as good and Vibrio infection in a open wound can spread much faster with carbon source.
On my phone at the River, sorry for spelling and punctuation. My Mom was a English teacher, me not so much
Good luck.
Good luck!
