This Haworthia isn't that uncommon. It's known simply as Haworthia 'Grey Ghost'. When it gets to the species name, it's often identified as Haworthia retusa 'Grey Ghost', but retusa has been changed to H. magnifica var. acuminata by Bayer, which would make it H. magnifica var. acuminata 'Grey Ghost'. Confused yet? Don't be. The naming of Haworthias is still very much in transition (as all botanical naming always is), and certainly at this point, they're all synonyms and can all equally be used.
Individual plants can vary a lot depending on the care they've received, especially the amount of light.
It may not be all that uncommon, but it can be a very impressive plant with it's whitish-gray variegation.