To get a sense of scale in these pictures, the dark blue tray with the water in it is about 5 inches across. The leaves floating in the water are tiny - only 1/4 of an inch in length. So, this worm is pretty small - probably less than an inch in length.
While looking at the worm a little closer, I noticed it had little tendrils coming off it in places and the tendrils were moving! I thought that was a bit odd.
Upon further inspection, I discovered those tendrils weren't tendrils at all, but were tiny worm-like critters (maybe a couple mm in length) that were swimming around! Ick! I'm really at a loss to explain what they are. After researching it a bit, I'm guessing these are some type of nematode. Most nematodes are microscopic, but some are bigger.
In these last two pictures, I highlighted the critters so they are easier to spot.
In this picture, some of them look a bit flatter - at least in the picture. When I was watching them with my naked eye they always appeared tube-shaped to me. Perhaps it's distortion from the water or they really are flatter than I thought. If that's the case, maybe they're planarians. I dunno.
I was surprised by how strong they were. One was flipping around on the edge of the water and was actually moving one of the leaves that was floating in the water. Pretty strong for something only a few millimeters long!
There's a whole ecosystem living in my bonsai plant of which I wasn't even aware.