The brown "stain" on a print can also come from inadequate rinsing during processing. The pattern of the area would likely reveal whether it's that or acid from the tube.
As far as removing mold from prints:
I have been happy with using 99% isopropyl alcohol on FILM. I soak a micro-fiber cloth in the alcohol, and rub the film well. Then I rinse using alcohol from a wash bottle. Let dry.
Because it has worked so well on negatives (and slides), I'd try using it on prints. But testing carefully, at first--perhaps on a border.