Whats wrong with my fish?

i have a platy i got almost a month ago and theres like a white chunk taken out of his "skin" ontop of him before the fin its almost like an indent. i also saw him scratching himself on a terra pot i have and in the right light you can see flaky things coming off the sides of him that are stuck to his skin. i have 3 more platy in the tank. whats wrong? will it affect any of my other fish? ps. all the platys i have in there went in around the same time and the only thing that the platys are doing are scratching themselves sometimes (which is normal i think)? help!