Cleaning stucco chimney

[johnha]

The wife is a bit perturbed about what my new insert did to the exterior of our chimney over the past year. (never had these issues with the SS pipe on the woodstove)

A reddish brown colored goop has run down. My 2650 PSI pressure washer did no good. Scrubbing with bleach did no good. Scrubbing with chlorine did no good.

What is this stuff? How do I clean it? Any way to prevent it in the future?

Thanks!