Looks like internal parasites to me. Best to isolate her before she infects others. I had a platy just like that, very thin, no desire to eat. Eventually it died. Found out later why... You can try Melafix though I've never used that before. What I use is a product called Tika and it's in a a small droplet bottle not unlike the EyeMo bottle. You can buy this at 88Marine.