Hi and welcome to the forum
Yes fish can pick on each other and bullies beat up other fish in the tank. There doesn't have to be physical interaction between them and quite often it is only a slight fin movement that can stress another fish.
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How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
What sort of filter do you have?
How often do you clean the filter and how do you clean it?
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What is the ammonia, nitrite, nitrate, pH and GH of the tank water (in numbers)?
Mollies need a pH above 7.0 and a GH around 250ppm.
Tetras come from water with a pH below 7.0 and a GH below 150ppm.
If there is not enough minerals in the water, the mollies will struggle to survive.
The GH and pH can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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Any chance of a picture and short 20 second video of the fish?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
If the video is too big for this website, post it on YouTube and copy & paste the link here. We can view it at YouTube. If you are using a mobile phone to take the video, have the phone horizontal so the video takes up the entire screen. If you have the phone vertical, you get video in the middle and black on either side.
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Do a 75% water change and gravel clean the substrate every day for a week or until we work out what is wrong. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.