Mold? Mold-like substance? Suspected organic microbial growth? Black fuzzy stuff? Unknown biological matter? Exactly what should we call this stuff?

I have come to the conclusion that there is no right answer. None. I'll go through a bunch of potential descriptions for this stuff, and I'll explain why there's a problem with everything.
Mold? No.
We don't always know that something is mold without testing it. I've seen many materials that I would happily bet my house on as mold, such as the stuff in the photo above. But other times, there can be black stains in attics that look like mold but are not really mold. So there's a possibility of being wrong.
Not only that, but using the word mold can also get lenders whipped up. I specifically remember one situation where I mentioned some mold underneath a sink and recommended cleaning it up.

The lender saw my report and made a huge stink about this. I can't remember exactly what happened, but I think the buyer ended up having to find another lender who didn't request a copy of my inspection report.
In this case, I created a bunch of unnecessary hassle for the buyer over something that amounted to nothing. After that incident, I decided that I was probably doing a disservice to my clients to even call something mold in my inspection reports.
Black stains? No.
After I decided to eliminate the word mold from my reports, I started calling it "black stains". My focus was always on fixing the cause of the "stains". I'd tell my client there was a moisture problem and I'd usually tell them why it was happening.
This worked fine for a while... until some contractor came out to fix an attic moisture problem that we had called out. The contractor who came out to fix the moisture issue expertly labeled the black stains in the attic "mold", and proceeded to tell our client that we were incompetent for not calling it mold. That led to a complaint call.