White stringy poop is a trademark sign of an internal parasite, and very likely Brooklynella. Take your clowns out immediately and place them into a QT if you've got one. Yep read again, BOTH clowns and/or all fish. Brooklynella is extremely contagious. Do a search on wetwebmedia for clownfish disease or broolynella and read up on the symptoms. Is your clown having a hard time breathing at all?
If Brooklynella sounds like the culprit, buy formalin and do 30 minute dips every other day no more than 4 or 5 times until the symptoms are gone. Keep them in QT for a month after all treatment and feed with good frozen food soaked in selcon.
