I have this problem on my sun, it was slowly dying off one polyp at a time. I think the algae have over power it n caused it to die off.
What I did, I cut off the base rock the sun is attached to. Then scrape off all the brown stuff. put the coral in a shaded place. Pray n hope it recover. After a few days, I saw some polyps open, I then spray some food to make it feed. After a week or more I saw it getting better. I took it out and feed each polyp with mysis. Now it is recovering, sweat!
Hope this helps.