[quote=@Jbcool]“Aye si-,” McNespey stopped and peered for a moment at the long-range scanners, rubbing the screen as if to remove a speck of dirt, then looking back to his commanding officer, “there's something else moving toward us, something smaller than an escort-class vessel, sir. It appears to be an [u][b]Emperor's Child[/b][/u] exploration craft.”[/quote]
Emphasis mine.

'Emporer's Angels' is just one of the chapter's nicknames from before the heresy. It references Fulgrim's influence on them to want to be perfect in everything, as well as the eagle's wing featured in their emblem. Admittedly, it's also been used on occasion as a generic term for space marines in general.

Ironically, the term likely wouldn't be used to refer to either the dark angels or the blood angels. The prior because they scare the piss out of basically everyone in the imperium capable of pissing, and the latter because they are the most respected and well-liked chapter among the imperial armed forces. It's well reflected in their chapter masters. Azrael is a scary mother-fucker who doesn't afraid of anything, and Dante... well, 1,100 years of dedicated service to the imperium speaks for itself.