Hi everyone! I’m new here. I noticed this grey discoloration on a mini Phal of mine, does anyone have any ideas of what this might be? They are in a south facing window with white frosted glass. The grey is more on one side than the other, as shown in the photos, and it is more on the side facing the window. Is it possible this is just mild sunburn? I’m worried it might be a fungus of some kind.