Why do they call him "Whitebeard"?
He doesn't even have a beard!
Edward Newgate's nickname in Japanese is Shirohige (shiro = white, hige = facial hair). Japanese does not distinguish between beard and moustache1. However, a literal translation White Facial Hair sounds a pretty poor nickname for one of the World's strongest pirates, and even sounds comedic. Whitemoustache isn't much better either. Whitebeard has a nice ring to it.
Also, fairy-tail's Lucy's nickname for the Celestial Spirit King is "hige-oyaji" (translated as moustache-man, in context), and he has only a moustache, which suggests that "hige" is used to refer to both moustache and beard in Japanese.
1: Google Translate says that "Beard" is "Agohige", and "Moustache" is "Kuchihige". However, they did not call him "Shirokuchihige" in the original, so the distinction seems to be not widely used.
Why is Blackbeard called Blackbeard when he only has perma-stubble? Whitebeard´s flawless mustache is manlier than a thousand normal beards.
And I quote:
A beard is the collection of hair that grows on the chin, upper lip, cheeks and neck of human beings.
A mustache is also a beard.
When the manga was being fan-translated, the translator left a note about how the name in japanese could be translated as white mustache or white beard. His words went something like, "Who wants to be White Mustach when you can be White Beard."