Stains from water creosote or soot leaking out of a chimney such as in this attic.
White growing chimney attic.
Flue gases and sparks can enter the building.
The stains tell us that water has been entering the flue raising an alert for frost damage.
It is important to notice stains on a chimney exterior surface.
This is not just a cosmetic concern.
The white material which appears on brick is a powdery mass of minerals called efflorescence.
To solve the issue you can reroute it to a vent which is going out of your attic.
And the rh from the grow tent air is very likely to get stuck with this heat and cause mold on the attic.
Efflorescence occurs when moisture moves through concrete or other masonry.
The following getting rid of mold cleanup methods are recommended by epa and all of them can be applied in the attic assuming that it is a small area up to 10 sq.
The moisture picks up minerals carries them to the surface of the masonry and leaves them behind in the form of tiny crystals on the surface as the moisture evaporates.