Dracula Lotax mark on leaf

[Longroots]

I've had this Dracula in semi hydro and its roots all seem really happy. However, today I noticed a weird mark on the leaf. I'll post a normal image and the edited image to see how the discolouration is forming. A similar thing happened on the tip of another leaf but it halted. I've not fertalized it and I'm using exclusively rain water. It's on a North facing window that pretty much had the East side blocked from light also. I've tried to make it as humid as possible by sinking it in very low in the semi hydro pot, but when I can afford to I'll likely be making an orchidarium for it.