Whats growing on and harming the leaf

I would of guessed some kind of pest though im not seeing any proper damaged leafs what i personally think has happend if you have fresh air intakes then most probably some kind of insect has got in tried to survive and died off either that or it could be a bit of feed thats go on the leaf in watering It could be a ph imbalance i would check your soil ph and see. If you see damage on the under side of leaves this often indicates some kind of pest so keep an eye on all the stems see if you can catch one or have a brush of the top soil and see if anything jumps about :) best of luck anyway my friend im sure its most probably dead by now lol