Fawns can actually eat themselves to death. You can use less powdered milk with the usual volume of water until the manure is normal again. Then gradually increase the milk powder back to the proper amount according to the recipe. If diarrhea continues, give a baby's dose of human medicine for diarrhea such as Kaopectate or Pepto-Bismol. Use the dosage directed by the instruction on the bottle.
Diarrhea is usually watery and yellowish from eating too much milk or from a disease.
It is usually green from eating too many greens. Female reindeer lick their fawn's rear end to do the same thing. If your fawn is older than three weeks when you start, it may not be willing to drink from a bottle. If it isn't, offer the milk replacement in a pan or bucket. After the fawn is about five days old, offer it some grain as soon as it finishes the milk replacement.