@NecRock:
It's not about the marines alone, he is known all over the world as a famous pirate hunter. Once you get a nickname, it isn't always easy to get rid of it. In the world of One Piece it might be a big question a decade or so after the story, why the first mate of one of the most notorious pirate crews in history was known as a pirate hunter, but for now, it's entirely logical.
And besides, a nickname doesn't have to be related to the current reality, does it? Because last time I checked, Edward Newgate didn't have a beard :p
Whitebeard is an incorrect translation.
His name in japanese is Shirohige
Shiro is white
Hige mean moustache, but can be used as beard.
Shanks has a tiny "beard" of sorts but he mostly has hair on his head and a little facial hair.
Blackbeard has a thin bushy beard.
White Moustache has a HUGE moustache.
And he was known as a Pirate Hunter for hutnign Pirates with bounties, but now he hutns other Pirates basically.
I'd like to see him gain a new nickname, sometihng cool like "Hawk-eyes" mihawk.