That's about what you'd expect with high nitrates and no available phosphates.
Nitrates kinda super-charges photosynthesis and actually raises the coral's demand for phosphates in more than one way.
Without PO4, you get lost tissue as the coral deals with the damage in a way other than repair.
If I didn't say it already, take any carbon dosing offline (vodka, "vsv", pellets....anything) as well as any other "extra" nutrient removers like GFO, bio-blocks, etc. ;)
Once those are offline, if you want you can wait a day or two to see if there are signs of recovery.....or if you have a phosphate fertilizer like Seachem or Brightwell available now, I think I'd probably go ahead and dose up your phosphates to at least 0.03 ppm. 0.10 ppm might be more ideal to begin with since there will probably be a surge of uptake once "some" is available. There's basically no risk to an overdose, so just calculate your doses and measure carefully as you would with anything else – no worries needed. Test at least 24hrs after dosing to see what's still left in the water, and re-dose as needed. You could also re-test in as little as 30 min to an hour if you wanted to see what early consumption was like and re-dose as needed then, but that's going above and beyond. ;)