Seeing this on your daughter's underwear is worrying, but a pale mark like the one in your photo is common in girls this age. It often comes from irritation by soaps, bubble baths or tight clothes. Wiping from back to front can also cause it. The skin there is thin before puberty, so it gets irritated easily.
Next Steps
Wash your daughter's private area with plain water only and pat it dry. Teach your daughter to wipe from front to back, and switch to loose cotton underwear. Please book a paediatrician visit in the next few days for a gentle external check.
Health Tips
Go the same day if you notice bleeding, a foul smell, pain or burning on passing urine, or fever. Also mention any worry about an injury or a small object, which the doctor can check calmly.