There are currently no alien visitors that would be taken as human by other humans. This myth was spread by government disinformation agents early in MJ12's history, as a means of raising paranoia in the populace. The distrust of aliens and their purported takeover agendas was to reach into every household, every family, and every bedroom. You, too, may be married to an alien. Movies were produced that showed humans being snatched and clones put in their place, and if not a replaced, then possessed by an alien soul or implanted with a control mechanism. All to raise the level of paranoia so that citizens could be expected to turn in their friends and neighbors if they mentioned having had contact with aliens. Of course, there are Star Children, who are souls from other worlds in human bodies, and a possession or a walk-in can and does occur, but all this is not new to human consciousness. Contrary to some of the disinformation, there are no alien life forms living in human society, as human lookalikes or otherwise.
It is frequently mentioned that the Pleiadeans look very much like humans, as well as the Nordics and Sirians. Where the similarities immediately impress humans having contact with these hominoids, there are differences that would stand out should they simply set up housekeeping in the neighborhood. The eating habits would be the first to be noticed, as where genetic engineering of hominoids has resulted in similar looking forms, the worlds the hominoids had to adapt to differed greatly. Nordics eat little, having learned to adapt to lean times on their world. Faced with what humans consider a meal, a Nordic would appear a picky eater. Just as the Men-In-Black are under stress when exposed to the Sun's rays, hominoid life forms from dim suns such as ourselves have difficulty dealing with a mid-sized sun such as yours. This would be true of Pleiadeans as well as Nordics, who would not simply get sunburn, but would develop symptoms of radiation poisoning as well.