Any chance there are any chemicals making their way into your tank? Windex etc. from cleaning nearby? I read one about someone’s lotion containing that chemical from sunblock that kills corals. Every time they stuck their hand in the tank they were slowly poisoning their coral.
if you’re not running carbon, now might be a good time. If you are, maybe replace it.
It looks like it’s on a frag plug - makes it easy to dip in Revive, or CoralRX.