Quintessential Deckerstar - S3-E23
Audio problem: Chloe enters the scene with her handgun drawn. She carries a Glock 19. Glocks are striker fired and have no exterior safeties to click or lower which would produce a sound, however when she draws and points her gun a click can be heard. (00:34:25)
Answer: It's Lucifer's blood from when his wings got hit by the bullets. As Lucifer spreads his wings, they're white, and before he wraps them around Chloe, you see a bullet hit his wing above his right shoulder, and the spot turns red. After that, he continues to get shot in the wings before fleeing.
Bishop73
Agreed. His wings take damage and "bleed" similar to how he bleeds when Decker is around.
Ssiscool ★
If they were normal bird's wings, the lower part where there are just feathers should neither bleed like that nor be able to stop bullets, but they are angel's wings, so they behave differently.