Oil furnace flue has green stuff dripping down?

I have a newer 2002 80% efficiency oil furnace. I do not know much about HVAC but I'll try to explain as best I can. A steel exhaust flue extends essentially straight up towards the roof. About a year ago a streak of "green stuff" began dripping down the outside of the flue. Just started using the heater again and noticed there's about 4 streaks of this "Green stuff". I finally opened the bottom flue cap to check it out. There was a lot of soot and I couldn't see the source of the "green stuff". It appears the green stuff is coming from between the inner flue and an outer steel tube. Does anyone have a clue what this is and how to fix it? The green stuff drips down and some has collected on the floor. It hardens and almost looks like acid or liquid mold.
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