Lancelot sees the flash of purple robes, out by the lake, and goes to investigate. He's wearing his sword, as he sometimes is when he's out wandering. As he gets closer, he recognizes that this man is dressed in similar fashion to Lan Wangji -- though he is clearly on guard, well-armed.

He rests his left-hand on the hilt of his sword, not ready to draw but an unsubtle reminder that the sword is there. He calls out, "Hail, sir! Are you in need of aid?"