Nephilim's spirits had not occurred to me as being part of the greater demon population, but you may be right. Sort of like fallen angels once-removed. However, Jude 6 [see also Luke 8:31] suggests they are no longer at liberty to be cavorting about masquerading as UFOs:
"And the angels who did not keep their positions of authority but abandoned their proper dwelling—these he has kept in darkness [zoφon], bound with everlasting chains for judgment on the great Day."
I don't think all the fallen angels are "bound." Some, including Satan, are free to move and have authority over Earthly kingdoms.
For example, consider Daniel 10:12-13 wherein the angel speaking to Daniel had to fight with an evil angelic being called the Prince of Persia and needed the Archangel Michael to help him.