I use baby tear-less shampoo to wash the face. Works great!
Ditto that and ditto not spraying the water hose in the horse's face.
If the baby shampoo doesn't get the MTG out, very very carefully use Dawn dish soap and warm water.
You will want to have your wash cloth good and wet but not so wet the water will stream into her eyes.
Use as little shampoo or soap as possible because the more you put in the forelock, the longer it will take to get it out