I know this all too well.
Droppings are made of: urates (white), feces (brown poop-ish) and urine. All should be present, but with a bird that is more frugivore- that can be skewed,
Do you have a photo? I can tell you approx how old your baby is...
If your mousebird is still alive- the most important thing is the formula consistency (dehydration and warm for digestion are the largest causes of death) and then nutritional content, I have a tried and true to those types of Coliiforme in an emergency:
1/2 tsp ground and soaked high quality cat food (as it has taurine)
1/2 tsp banana (potassium and magnesium)
1/2 tsp carrot puree (vit A)
1/2 tsp hard boiled egg YOLK (13 essential vitamins and minerals- including D)
1/2 tsp quinoa flour (Quinoa is gluten-free, high in protein and one of the few plant foods that contain sufficient amounts of all nine essential amino acids.. It is also high in fiber, magnesium, B vitamins, iron, potassium, calcium, phosphorus, vitamin E and various beneficial antioxidants)
1/8 tsp peanut butter (thickener- B6)
1/8 tsp apple sauce (helps crop mobility/sour crop)
Add a bit of apple juice until it resembles baby food consitancy (depending on age)
sprinkle of probiotics and bird vitamins (that includes calcium)... this fills in the gaps...
People worry about iron.... dont worry about that. It is very rare that iron has an issue- only if it has excessive vit C.... so relax and enjoythe experience of raising a baby. Otherwise i just use a normal formula mix (katee or other) with added babyfood puree with fruit.
Your baby should be putting weight on daily... before the morning feeding, use a gram scale.
1-2weeks old 1cc every hour
2-3 weeks 1-2cc every 2 hours
3-4 weeks 2cc every 2 hours with some soft fruit bites
Try to wean..... you will find this very hard.
You can PM me, or respond and i can give further instruction.