| It's a bacterial browning which is almost always (at least in Paphs) caused by water sitting in the crown usually combined with night-time or cool conditions.
The leaf should be removed but you will need to be very careful. You might find if you tug gently on the leaf it will come out without resistance or you may need to cut as much of the damaged leaf off as possible. I then recommend treating with cinnamon or Physan.
You may loose the whole growth but if caught early enough it may be ok.
It's always best to try to prevent such things, in this case by making sure there's good air flow around your Paph, to try to avoid getting any water in the crown, and to water early in the day, preferably morning so the growth can dry off before night.
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