THERE'S mention of "unclean spirits" in the New Testament (Mark chapter 5, Luke 11:24 are two examples).
How are these different from "evil spirits"? I'm not sure of that.
However, I am certain that "unclean spirits" are NOT any of the evil dead who have returned to earth.
Yes, the spirit world may also be on earth, but it is in a different dimension.
Also, when an evil (or unclean) person dies, they are in spirit PRISON.
A prison denotes confinement, being bound. So, how does an evil person die and return to haunt their house, or possess the body of a living person? They don't.
(Yes, men may sometimes escape from the prisons of men, but I'm sure God's prison is escape-proof ...)
Repentance is the only possible means of escape from spirit prison.
On the other hand, the one-third of the host of heaven who were cast out in the pre-earth life for rebellion are loose all over earth, tempting us and are eager to possess even in a swine's body for a brief time.
These evil spirits have watched the lives of people, so they can impersonate them, perhaps even mimic their appearance.
If the righteous dead can only interact with mortals on very special occasions, then why can the evil dead run loose on earth? No, that's not consistent with an orderly plan of God.
(Such a scenario also unbalances the "temptation equation," if the evil dead have such power ... and the righteous dead do not.)
When Ted Bundy or Hitler died, they went to spirit prison and are confined there, tormented by their own heavy weight of sins. They are NOT roaming about the planet tempting, impersonating or possessing anyone.
Yet, I don't doubt that those 1/3rd of the host of Heaven cast out are eager to impersonate Hitler or Bundy to fool, confuse and further torment, afflict and lead mortals away from the truth of the Gospel.
It is these 1/3rd evil spirits who keep getting mistaken for the real deal -- the departed dead -- and they probably get a thrill out of that deception.
-This, of course, also means there are no REAL ghosts on Earth. No one who lived before as a mortal haunts any place. That is all done by impersonators, whose entire existence resolves around deception.
-So, it is looking more as if "unclean spirits" and "evil spirits" are pretty much the same -- both of the 1/3rd of the host of heaven who were cast out.
The Biblical translation is confusing at the least in that regard. Too bad the Prophet Joseph Smith didn't have time to finish his revision of the Bible.