All bird droppings are made up of three parts: Feces, the solid, central part which can vary in color depending on the food the bird eats. Urates, the next layer of the ring, which can be cloudy-clear or with shades of white, yellows and greens, again depending on the foods eaten. Urine is the clear liquid, usually outer layer of the ring. Depending on the amount of fruits and fluids the bird eats/drinks, this can be a significant part of the dropping.
Diarrhea is often actually Polyuria. True diarrhea is when the feces part of the droppings are not well formed and liquidy. Diarrhea is usually caused by a disease. Polyuria is when there’s more than usual amounts of the urine and urates (the feces are still well formed).
Polyuria may be caused by viral infections, allergies to foods or even a tumor somewhere. There are just so many possible causes that a vet visit is essential in order to catch things early.