I think that’s just a really bad case of ick. I would use this stuff to help your poor fishy.
API MELAFIX Freshwater Fish Bacterial Infection Remedy 4-Ounce Bottle https://www.amazon.com/dp/B0002567SM/ref=cm_sw_r_cp_api_i_kkcDEbNR13J5E
Me and my pa use it all the time and had a ton of luck with it in the past. I would suggest that u put your fishy in quarantine in a smaller tank, add a heater and make sure u keep things clean. Let me know if u need some more help. Also full disclosure I’m no pro but I do know more than most when it comes to keeping fish.