I've heard sometimes dosing iodine can help prevent bleaching at the base, I'm not sure if there is any scientific basis behind it but I have heard it from a few people including a video on Mike Paletta's elos tank.
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I would cut and remount. Once the tissue dies it doesnt come back
With the picture the way it is its impossible to tell you what to do - the 'dead' area is totally in a shadow and the whole picture is out of focus.
that does indeed look necrotic