Hi guys I just joined up, mostly because of one little orchid doing something interesting.
I rescued 3 orchids from a friend's place where they were under-watered and over-sunned, about 2 months later they are all fine, growing new roots and leaves.

However, one has developed this rot/cellular breakage or something on the end of the new leaf. The other 2 are totally fine, no one else in the house is having any issues except my jade across the room.
I've looked at various images of diseases and such but I still haven't figured out what it is... I just wiped it with peroxide to be sure... and it's gotten darker from yesterday. Does anyone know what it is and what to do?
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Thanks!