Hi and welcome to the forum
The fish has been bashed and has a bacterial infection. Get a broad spectrum medication that treats fungus and bacteria and treat the tank.
Do a 75% water change and gravel clean the substrate before treating. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media in a bucket of tank water.
Remove carbon from the filter before treating otherwise it will remove the medication from the water.
Increase aeration/ surface turbulence to maximise oxygen levels when using medications.
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If you have 10 mbuna cichlids in a 20 gallon tank, they will eventually kill each other. If you can post pictures of the fish, we can identify them for you.
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.