Leaf spot on Aerangis

[shakkai]

Hi everyone, While soaking the mounted plants today, I noticed a problem with my Aerangis mystacidii x modesta. One of the top leaves has this 'spot' where the leaf itself has gone very translucent and the back of the leaf is turning brown. I've never seen this before - but then, I think I've been lucky so far with my plants!

Can anyone tell me what this is? And most importantly, what do I do about it?!

Here is the front view:


And here is the underneath:


Apart from the orangy-brown inside the box I've drawn on the photo, the base of the new leaves and the tips of the roots have what appears to be a natural, orangey tint to them. So far, the 'damage' is confined to that one spot on that one leaf.