Would you be able to upload a photo of the tank please? It can help
What sexes are the mollies, and the platies? If you're not confident about which they are, and it can tricky to tell with mollies especially when they're still young, then uploads photos of them that clearly show the anal fin, and we can tell you if they're male or female.
It can happen. Livebearers are generally peaceable, but there's some personality variation, and you get the odd one that is just... mean. One of my male platies is a strutting peacock of a fish, and very protective over his ladies. If he thinks another fish is getting too close to his girls, he'll chase them around a 55 gallon, his fins all stiff, looking quite proud. But he doesn't get a chance to target just one fish, so the chased ones don't get exhausted and stressed out. So yes, it's not that unusual for some chasing to go on, or even courting behaviour from a platy or a guppy towards a molly, but it's not good either for the one fish to be constantly targeted, and we need to figure out why and how to fix it, or the exhaustion and stress could kill her. Ripped up fins can also lead to bacterial infections, threatening her life too.
There are a few possible reasons it could be happening. If there is the wrong ratio of males to females, they can act more aggressively than is normal. The amount of other fish around to sort of 'spread out' the bullying so it's not so targeting can be a factor. A tank without enough plants or decor to provide hiding spots and for fish to establish their own little territories is a big one. No matter how big the tank is, if the platy is at one end and can look to the other end and spot the molly without even trying, then he's more likely to notice her, remember that he's not happy with her for whatever reason, and keep chasing her, without her being able to hide, or him being distracted and giving up the chase. This is the first thing I'd suggest to do. Add more driftwood, stones, decor, and either plastic real plants. Anything that fills the tank up a little, provides hiding areas, and breaks a fishes line of sight.
Do you have a quarantine tank? Or can you set one up from a plastic storage bin? The little molly might need to be quarantined to give her time to recover while you sort out the tank, and if she develops a bacterial infection from the torn fins, you'll be able to medicate her without having to buy enough medication to treat 75 gallons of water. Much easier to medicate a ten gallon tank or storage bin.
