Thank you for the question.  Nice Picture!  I would need to have a sample brought to the office so I can identify the growth.  It seems to be a mycelia (fungal) growth of some sort.  I have not seen one like you have in the picture before.  I can only assume that it would not be a permanent problem and would remain gone once wiped away from the leaf.  Moist and cool environmental conditions bring out many new fungal growths. 

If you would like to bring in a sample or two to our office, I will more that happy to take a look at it under a microscope.  Thanks for the question.