So why not call them X-MEN?
EDIT: Just noticed the post above! Yeah, was wondering if they were d-level guys.
They're not literally X-Men, which are humans who spontaneously mutated (and the why of that has changed over the decades, first it was because of residual radiation from nuclear bomb tests, now it's because humans were experimented on by an ancient progenitor race to give them latent genes that only unlock in certain individuals, explaining other metahumans as well as proper mutants).
The Inhumans were a group of humans taken by aliens early on and experimented on to react to a certain specific mutagen (Terrigen Mist) which would give them powers, and maybe some other benefits i don't know about. These humans were then transplanted to what is called the "Blue Area" of the moon, an area of Earth's moon terraformed to be habitable.
A closer analogy is with the Atlanteans than with the X-men, really, the X-men bit just better describes how their powers work.