I've seen a few of my fish have stringy white poop on occasion and nothing ever came of it. I don't think it's anything to be to concerned about unless it's accompanied with other symptoms.
It may be
nothing to worry about but it may also be your first indication that your fish has bloat.
It's a tough call as to whether to jump right in and medicate as soon as you see the long stringe feces. Fish can occasionally have this condition and not develop bloat however if you wait to see if other symptoms develop, (lack of appetite, lethargy, gasping, etc) you run the risk of being too late with the treatment. At the very least the presence of long stringy white/clear feces should put you on 'high alert': test the water, do a partial water change, siphon the gravel, make sure you're feeding the proper diet, etc and watch and make sure that all fish are eating at every feeding and swimming normally around the tank.
To answer your question, natalie62, on the Epsom salt: go with 1 tablespoon per five gallons of water. Dissolve it first and add is slowly. Epsom salt is NOT a cure for bloat but it does act as a mild anagesic and laxative so if bloat is being brought on by a digestion problem the Epsom salt may help.
As far as not feeding the fish while treating--it's not going to hurt them to fast for a day or two and again, if the bloat is being brought on by a digestion stressor then fasting may help. You can continue to feed if you want to but I'd feed on the light side until you're in the clear.
It's a really good sign that your fish is still showing an appetite. Finish out the clout treatment and on the main tank try upping the frequency of your partial water changes for the next several weeks. 2-3x a week @30-40% per change would be good.