The Great War between Immortals and Devils was still fresh in many people's memories. The times were more unsaturated and realms had only recently been made. Before the creation of such realms, devils, immortals, and mortals used to live together. When these entities separated after the war, it was common for other entities to keep tabs on other realms.
What happened in the Underworld traveled to the Mortal Realm and the Heaven Realm. Similarly, what happened in the Heaven Realm traveled to the Mortal Realm and the Underworld. Nothing was a secret, including the extermination of the Hua Clan, the last missing Princess of the Hua Clan, and the birthmark that differentiated the Hua Clan from other devils.
That's why, seeing the glowing birthmark on Xiaoling spooked the bandits and led them to create distance between them and Xiaoling. After all, the Hua Clan was revered as the strongest devil clan in the Underworld. No one wanted to offend them.
