I would use a very gentle flow to wash away the dead tissue. I can't emphasize enough "gently". A turkey baster would be too strong. If it doesn't come off with a gentle flow, I would just leave it and let the inverts separate dead from living.
In my experience, doing nothing is usually better when it looks that bad. Most actions just lead to an accelerated rate of death. But I've also seen coral rebound from terrible damage on their own!
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