My DD had nasty, mucousy poops on and off for months. She developed pretty nasty diaper rash from it, too.
The poop sliding from one side of the diaper to the other sounds like exactly what my DD had. [When she did actually have diar. from a virus, it was much less slimy and mostly just like... particles of poop mixed with poopy water. Yuck yuck yuck.]
My conclusions: mostly, I think it had to do with teething. I also suspect her early vaccinations, as the very worst episode (complete with spots of bright red blood either from immediately within or outside of her very, very sore rectum) occurred immediately after her four month shots. She had a lot of symptoms of dairy intolerance, but I never eliminated it (
) and the mucousy poop/diaper rash eventually went away. Sometimes it seems like dairy is 50% of her diet, and she shows no intolerance now.
Even looking back, I'm not sure what was going on. The onset occurred around her vaccines, but so did teething. Her improvement began when I stopped the vaccines, but I also started probiotics at that time.
W/ the next baby, one reason I plan to hold off on vaccines is just so that I have one less variable to consider when troubleshooting these problems that they have...