I think it is scale rot? Looks yellowish coming from the scales but I would confirm with a vet. Please keep in mind reptiles are very resilient animals showing no sign of pain until they are too sick to fake it anymore.

Please post the info about your tank set up so everyone can try to help you.

My suggestion is to get your pet back to good health first before any further attempts at switching to f/t. Feeding it live until it gained back most of its weight plus fixing your tank set up (Again, please provide details), and maybe get a vet to take a look at that belly.