It is called Yerba Buena in Cuba (but this name is used for multiple plants around the world).
In Cuba, yerba buena generally refers to Mentha nemorosa, a popular plant also known as large apple mint, foxtail mint, hairy mint, woolly mint or, simply, Cuban mint.
Source: http://en.wikipedia.org/wiki/Yerba_Buena
The specific plant seems to be:
Mentha × villosa (M. spicata × M. suaveolens) - Also called Mentha nemorosa, large apple mint, foxtail mint, hairy mint, woolly mint, Cuban mint, mojito mint, and is known as Yerba Buena in Cuba.
Source: http://en.wikipedia.org/wiki/Mentha
If you're in Cuba, you're likely making your Mojitos with Mentha nemorosa, aka "Cuban mint." The closest we can come to this, and it's very close, is Mentha spicata, aka regular old spear mint.
Source: forums.egullet.org