Q:

Why does Nightcrawler get called "elf"?

Why does Nightcrawler get called "elf" by the X-Men?  I thought he was a mutant human like the rest.

A:

The actual nickname (given by Wolverine) was "Fuzzy Elf". The reason was (from "Appearance" in http://marvel.wikia.com/Kurt_Wagner_%28Earth-616%29) :

His body is almost entirely covered with a fine indigo-colored fur.
Source: 

It was Chris Claremont's scripting that evolved him into the swashbuckling hero he is today. Claremont stated that he took the idea that Nightcrawler thought "it was incredibly cool to be blue and furry." 

Nightcrawler has pointed ears 

Here's a card where the "Fuzzy Elf" nickname is stated (if http://marvel.com/universe/Nightcrawler is not enough of a source):

A:

It's because he has pointy ears.  Elves have pointy ears, hence the nickname.  But it's strictly a nickname; he is a mutant human, not an actual elf.

