Store bought feeders (as opposed to home raised feeders) carry a serious potential for introducing IPs and other pathogens. One of the causes of non pathogenic white, stringy feces is the shells of shrimp, krill ect. IME white stringy feces in conjunction with live feeders (fish) is usually caused by IPs, without the benafit of a microscopic fecal examination I feel it is better to error on the side of caution. Treating with epsom salt as discribed above is and effective way of dealing with this. The epsom salt treatment is less harsh than most OTC meds and very effective course of action while the fish is still eating. Metronidazole and Praziquantel are two readily available medacations that can be us also. There is a medacation availabel produced by API called General Cure that is a combination of both medications. I would suggest to use the epsom salt first as this should solve the problem. I would strongly sugest you discontinue the use of feeders and get the fish on a high quality pellet. Also you should with hold feeding during treatment.
