What is it, is it fatal? How do I remove it. Or is it just old leaves?
It is a fungus (or mould if you prefer.) Not really lethal unless the plant is weak to begin with. If you grow Utrics then this will pop up from time to time. Never found any rhyme or reason for why. The powdery part should just wipe free.
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U. dichotoma seems especially prone. I'd get the mildew on the scapes routinely. Doesn't seem to much affect things other than the scapes wither a bit sooner. I'd submerge the plant for a week or so just to be on the safe side.
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