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This happen to me when I over use gfo to reduce phosphate to slow down algae growth.
I end killing lot of sps and I will never use gfo again.
Now I use turf algae scrubber and no more burn tip on sps.
This looks like 'growth' - but the picture is not great. I wouldn't go changing anything (GFO, etc) until you're sure what it is. Does it look like there is still coral tissue on the tips?