If the fish is not eating, medicating the water is your only choice. Saltwater fish have to drink constantly to osmoregulate so medicating the water is an effective measure.
White stringy poo can be caused by stress, intestinal bacterial infection, and secondary infections due to intestinal parasites.
Metronidazole and Praziquantel should treat the bacteria and possible intestinal worms. API General Cure contains both of these in the right ratio. If you don't have it or can't get it locally it should be available online but delivery may take a week or two. Maybe reach out on your local forum if some reefer near you has some to spare.