Hello! So, for future reference, the more detail you give on your tank, the better we can figure out the issue. In particular, we need to know your water parameters! Especially ammonia/nitrite/nitrate. High levels of any of those can often be at the root of any fish health problem.
Do you know the gender ratio of your platys? The injured fish in the picture is a female, and I just lost a female due to a similar problem, caused by stress from having too many males pestering her. It's best to have at least 2 females for every male (the more females, the better).
If you have a separate tank you can move her to, that would be a good idea. If not, then treat the main tank. I recommend using 1 teaspoon of salt per gallon of water. That concentration of salt should be effective at treating a wide spectrum of diseases, so anything that might take advantage of the injury will have a much harder time. Beyond that, just watch the fish and see how she does. If it gets better, great! If not, we can try and come up with something else.
One last thing: It's a bit of stretch, but some plecos have, on occasion, been witnessed attacking the sides of other fish. It's definitely uncommon, but I would keep an eye on him nonetheless.