What is this?

[Yoon jin]

This parasite-like thing suddenly appeared in pacman tank.
My tank was 35*35*45(cm) tank and half-water, half-land. My pacman went into the water occationally(once a week?), but it didn't poop in the water. Actually I have not seen it poop. I change its substrate every week because it can poop on the substrate.(I might not discover the poop because my substrate's color is very dark) Anyway, I didn't have water filter at that time so I changed half of the water every day(I have filter now). Then this parasite-like thing suddenly appeared before two weeks. This creature only appeared in water, not in substrate. Thus, I removed all water in the tank that day and let the water-part of the tank dry for 2 weeks. Then, today I discovered this creature in my guppy's tank!
My question is..
Is there a posibility that this creature is came out from my pacman?
What is the name of this creature?

this is a picture of my guppy tank. This creature stick all over the glass!

[GermanEdam]

Those look like flatworms, a naturally occurring worm that eats extra nutrients in your tank. They are absolutely harmless, however you should make sure that they do not have a triangular head under a microscope, a triangular head could mean they are planaria. If they are flatworms, they will go away when nutrients in your tank are balanced.

[Scales and Tails]

I had the same thing in my tank, they are harmless worms called detritus worms, they clean stuff up and are found in every body of water although large numbers like in your case can cause some concern. It's usually points to a hygiene problem like extra food or nutrients in the water. They should go away soon - like in my case if not look into potential causes of such a large number of detritus worms - left over food, dirty filters etc. Hope this helps

[Scales and Tails]

I'd also like to mention that it's pretty much impossible to get rid of them all if your considering trying that (I know I certainly wanted to find a way to kill them all ) they just live in the substrate eating debrey (although it's normal to see a couple on your glass here and there). I know they look off-putting but they are helping you out by cleaning up. They pose no harm to you or your fish, just wait it out and make changes if you think tank hygeine can be improved and they should go back to normal numbers.

[Yoon jin]

Thank you so much! It is good to hear that this worm is harmless. I will have filter in my tank today

[Yoon jin]

Is planaria harmful for my pacman?

[daybr4ke]

I've never had to deal with these myself, so I'm not 100% on whether its harmful or not, but a search indicates that they aren't very harmfuI. I remember users on here and dendroboard using panacur to get rid of worms. I looked around, and it appears to be frog safe. There's a liquid and a powder version. I dont know if it'll be hard to find in China. I hope this helps, good luck!