looks like a prolapse.
Do you have unscented Epsom salts on hand?
I would give this platy a daily bath dip in an epsom salt solution.
1 tablespoon per gallon of water. In a separate tank. Have this fish in it for up to 30 minutes each day. It helps reduce swelling.
Unfortunately if this does not work, the platy won't make it and may need to be euthanized depending if it doesn't improve
And platies are tropical fish and even if not tropical you are welcome to post about non tropicals all you want too