This one is more of a translation problem. In Japanese, his nickname is ''Shiro Hige''. The word "Shiro" meaning white, and "Hige" meaning beard or mustache. As seen on his face and his flag symbol, the man doesn't have any beard, but he does have white mustache. A more proper nickname for him should be "White Mustache". However, this name doesn't have a 'ring' to it as a pirate's name. This might be the reason why most people translated it as "Whitebeard" instead.
Now, if you check the dictionary, the meaning of beard is:
1 : the hair that grows on a man's face -often- excluding the mustache
So, technically this term still works, though prople don't normally use the word "beard" to refer to mustache.
I'd been wondering about that.
Also! I think they're just too lazy to give Zoro a new nickname. They were too busy trying to quickly finish a Sanji sketch! >_>