Long stringy white poop

[ALang]

Hmmm.... my Indian yellow-tail angelfish (Apolemichthys xanthurus) is a cute 4", 3 year old in my established tank. Eats like a piggy. Lots of mysis shrimps, nori, pelleted fish food. He gets food at least 2 times a day.
Today, I noticed that he has a long, stringy, white poop hangong out of his vent. Does anyone know why? Water parameters are great as in a reef tank.
Thanks. Lenny.