It looks to me like your fish is in the late stages of Septicemia. (possibly some kind of Viral hemorrhagic septicemia?)
This is a very sad sight to see, and unfortunately it is most likely going to die.
Sorry, but I have to agree. I have seen this with an endli that I had shipped from CA. Was advised to place it in a hospital tank (I already had an established one.), up temp, add salt, and perform water changes. All to no avail. The disease advanced very quickly, but it took several days before the endli perished.
Still I hope the best for your poly.