Furnace chimney leaking

I recently had my furnace cleaned by ABT, and the technician pointed out to me that something is leaking from the duct that is going into it. It’s some kind of white residue. See attached 3 photos of exterior and interior of the areas affected. I took this photo after he already cleaned but he said there was a lot more build up inside the furnace area shown. What could this be, and what kind of service do I call to help look into/resolve this? Thanks!