She should be on a waterfowl starter, geese have a high demand for certain nutrients not available in grains. Theeals may be helping but she will still be behind on trace minerals. On a rich but inadequate diet, they grow too fast and can suffer from joint degeneration and fragile bones. The white poo may because she is not eating. She may also have an infection, and the stress of a heatwave pushed her over the edge. For now, confine her in a softly padded container and feed plenty of greens and try to find waterfowl feed. You may want to take her to a vet familiar with geese, in case there is more going on. The vet will want to check droppings so take a few with you ( I am sure she will provide a fresh one) They can recover, it does take some time and occassionally the legs or wings will have deformities.