Stress can cause white poop. For a fish, stress is bad water quality or changes in water parameters. Measure for ammonia, nitrite, nitrate, and pH. Check the temperature. Aggressive tank mates may also lead to stress, as can illness and medication. Moving to a new tank or the addition of new tankmates can also cause stress.
The parasite hexamita, that can attack internal organs, very often causes white stringy poop. Feed the fish anti-parasite food that contains metrodonidazole.
Bacterial infections can lead to stringy white poop. Before acting check for other signs of infection (redness, lethargy, lack of appetite).
Info copied from:
http://thegab.org/Illness-and-Treatment/what-is-wrong-
with-my-fish.html#WhitePoop 
