There are others here who will be much more knowledgeable about disease. I will say it looks like a burn or serious scrape. It could be a burn from a coral or anemone. Is there a heater in the display? How is it acting?
No heater in the display. I was thinking nem or LPS burn. It's acting fine. I was shocked when I saw it. It was almost like it was stopping to show me.
This happened to me recently, two times in the past 3 weeks and it went away over night and or in 2 days. Mines is eating normally and back to regular health now
As mentioned above, it sounds more like a coral sting or injury of some sort. Meaning, it should heal without any intervention on your part. However, maintaining pristine water conditions and feeding vitamin enriched foods always helps.