Hi, 
Sorry but that photo is not in focus.  I suspect insects sucking it but I need good pics.  Could you send a pic of the sap and also of the mold?
Thanks,
Barb  

Hi, yes, those are good pictures. The leaves seem to look normal and I don’t see any insects although I am just seeing small parts of the trees. I have forwarded your pictures onto an arborist who is a friend of mine and I will let you know what he says.

Hi,
From my arborist:  your tree is a magnolia, not a lilac, and that is magnolia scale "which is at epidemic proportions locally. I have had more calls about it this year than ever before. Treatment can be challenging and, unfortunately, no quick remedies at this point in the life cycle."
Here's a good article about it so you can read about it and possible treatment options:
https://hort.extension.wisc.edu/articles/magnolia-scale/
Let me know if you have any other questions,
Barb