Sometimes white poo can just be a sign of indigestion. But if it goes on more than a few feedings though, it's very possible if not likely the fish is showing signs of Hexamita, a parasitic infection which can often lead to HITH (hole in the head) -- and which FH's are very prone to unfortunately.
As ny.subway notes there some good threads on MFK about this and how to treat it, here's one of the most useful by far (Sticky posted by RD):
Treating Hexamita aka Spironucleus with Epsom Salt Solution This is a rather safe way to treat any newly imported fish, as a prophylactic, just as one would use a de-wormer. It's not only an extremely cheap way to treat fish, the active ingredients are readily available world-wide, and it's...
www.monsterfishkeepers.com
Here's the main advice for initial treatment, using magnesium sulphate (Epsom salt, which you can get at most drug stores, just make sure it's pure, not scented like for that used in bath tub Epsom salts):
It's best to do this while the fish is still eating; afterwards it gets more complicated. So at the first signs of any white stringy poo I would suggest feeding pellets soaked in Epsom salt as per above -- even if your fish didn't have Hex this treatment wouldn't have any adverse affect, it'll help clean out its digestive organs.
If the white stringy poo didn't go away after doing this for 3-5 days you'd probably need to consider stronger meds like Metronidazole, but as your FH just started showing this symptom then maybe it hasn't gotten to that stage, so the more natural Epsom salt treatment is preferable.
Again it could just be indigestion, and not ongoing, let us know how it goes.
