Black worm in quarantine tank?

[LadySundew]

Here's a link to a video I took of the creature on a USB "microscope" camera. The quality isn't the best but you can see the shape an how it moves as well as the colour:

Sorry about the wobbling of the camera, it's kinda hard to navigate your target with it. I can upload more footage if needed. The worm is about 5 mm long.

Backstory:

My quarantine tank has been running for a few months now and I've gradually bought plants for it. With plants came multiple types of basic "pest" snails that I welcomed as cycling help. The tank doesn't have any other animals yet. Today I separated my clump of moss to tie it to rocks in a bucket. After that I looked into the bucket for snail babies to return to the tank. I found a few and also spotted some seed shrimp. Then I saw the worm. I scrambled for my pipette and quickly captured the creature. I first took it under my regular microscope but had trouble really seeing it since it's alive and moving so I switched to the USB camera.

I've googled a whole bunch but can't figure out what it is as I haven't found a perfect match. I also found one similar worm in the tank but it was white and got away when I tried to catch it. Weirdly I spotted creatures that I had thought to be seed shrimp but now noticed actually were copepods?? That's not a concern for me but I just found it weird, like where did those come from? But the worm I am slighty concerned about since I am planning to get shrimp for the tank where the plants are going to and my understanding is that Planaria can harm shrimp? I think this worm does resemble Planaria but I'm not at all sure. So, is it Planaria? Or something else?

This video might actually be a bit better:

 

[StarGirl]

Looks like a leech to me.

 

[LadySundew]

Now that you say it, it seems obvious! It does look like the bigger ones I've seen in ponds and moves like them as well. I googled again and it's definetly a leech, the shrinking and streching movement is unmistakable! It seems that they can be dangerous to multiple animals depending on the type. I'll keep an eye out for them, if I see more of them and keep on googling. Thanks for responding!

 

[Feohw]

the shrinking and streching movement is unmistakable!

I don't see this in the video, or was that movement just not captured here? Leeches shouldn't glide along like this, they stretch out, clasp on with a sucker and then they pull their body forward, often compared with the movement of an inchworm. This one is moving like a flatworm. Just looks like Rhabdocoela too. Most people associate Rhabdocoela with the whitish/clear worms we usually see, but others look exactly like this.

 

[LadySundew]

Well I guess the movement wasn't so unmistakable... I'm real confused since I swear I saw it doing that streching and unstreching while being attached to the bottom of the container. Like it was swaying itself around? Anyway, I went trough the videos and observed the thing since I still have it and I see nothing like that now. Maybe it was simply staying in place and inspecting its surroundings? I went ahead and took a new video. I caught another "worm" from the tanks wall today and put it with the earlier one. Both appear in the video. The new one is smaller and lighter in color but otherwise I think it looks the same. It looks yellow on the camera but on the tanks wall seems white and quite alike the pictures of Rhabdocoela I looked up. Also the old one looks injured somehow, maybe I hit it while adding the new one in. I kind of feel bad for it but it seems to be lively still.

So now I would be certain that these are Rhabdocoela but I also felt certain about it being leeches so if someone might back this identification up I could relax since Rhabdocoela seem to be deemed harmless.

 

[Feohw]

Yeah, those are definitely Rhabdocoela, not a common one either. If the appearance alone isn't enough to determine that, the movement sure is. So yep, certainly nothing harmful. Flatworms can move their bodies in pretty funky ways, so that would likely explain why it confused you at first.