I've cleaning up old saddles for a little while now. My favorite thing to do is cleaning the ones that have been left to collect dust. So I've seen alot, but that is not mildew or mold. U have a stain in the leather. It may or probably wont(saying that bc it's really really dark). U can try to darken the saddle (olive oil will not darken leather-unless it is very very dry-then the leather is actually lighter than it originally was, and adding olive oil brings the color back it USED to have)... BUT for anything like that to work, u will have to go AROUND the stain, otherwise it will darken also, and still be darker than the surrounding leather. I wouldnt worry about it really. It is in a hidden area, it doesnt effect the function or soundness of the saddle.
Mildew and mildew is whitish and sometimes even has a green tint, AND it have a rough texture. U can scrape it off with a fingernail. Mold will sometimes be black. But it still has a grimy rough texture.
Angels are people who can see and feel the pain in all creatures... -myself