Hi and welcome to the forum
I can't tell what sex the fish is because it is holding its anal (bottom) fin against its body. Its tail is also chewed up a bit. If you post another picture with the anal fin standing away from the body, we should be able to tell the sex.
Female livebearers (platies, guppies, swordtails, mollies) have a fan or triangular shaped anal fin. Males have a long thin anal fin called a gonopodium.
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You should monitor the fish and see if it rubs on any objects in the tank. And if it develops a cream, white or grey film on its body then check the water quality for ammonia, nitrite, nitrate, pH & GH (general hardness). Post the water quality results here and another picture of the fish if it gets patches on it.
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If the tank is newly set up (less than 2 months old) reduce feeding to a couple of times per week and do a 75% water change and gravel clean the substrate 4-8 hours after feeding. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
You should also do a 75% water change and gravel clean the substrate any day there is an ammonia or nitrite reading above 0.
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edited to add, the pic of your fish in the other post you did shows the fish to be female and she is pregnant.
watch the black widow (skirt) tetras because they could be taking bites out of the fish's tail.