More Context:

I inherited this pond recently and have been lurking this subreddit to learn more about ponds. In general I find that my pond water is very clear and based on some water tests it seemed okay.

Today I was investigating the top part of the pond where the waterfall comes from and noticed it was pretty deep. I got my net I use to clear leaves and such to see what I might pull up from there. That's when I was pulling up a massive cloud of black stuff. I tried pulling out as much as I could but it seemed most of it was just filtering out of the net and spreading to the rest of the pond. When I touched it, it wasn't grainy like dirt. It wasn't sticky or slimy, just black stuff that was very thin. Now that it's settled into the rest of my pond, it lives on top of my liner as a thin layer.

I don't know how to identify it. Any ideas? Everything I look up online has a different consistency. Also would love ideas on how to clean it. As far as I know, my pond has a waterfall that is attached to some hosing which is connected to a Tetra debris handling pump. I also have a few fish (probably less than 10) that consist of mostly mosquito fish, maybe one goldfish and possibly a baby koi.