The fish has some fin damage..probably from handling and nets.
So if I understand this. The wasnt eating for several weeks so you started to force feed it... yesterday it ate a little. Thats encouraging.. I would separate the fish and let it eat as much mussel as it wants then intro other foods.
I would stop force feeding at this point.
You want the fish isolated so you can be sure its eating and doesnt pass off colored feces.