Christy, I suspect the tissue is still there. If you see no further recession and/or tissue actually falling off, you can leave the "dead" part. If there is a pathogen causing this, you want to cut it off from reaching live tissue.
However, I think you're OK just lowering it a bit and watching. I think the tissue is still intact. If it stays white for more than a day or two, the tissue is for sure still there....no tissue = diatoms.
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Brad
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