All I know is that Captain Kirk and William Ryker got it on with loads of aliens, some of them were hot, even if they were green. Whereas as other less desirable aliens, might in fact come across as scary looking.
However, if these aliens looked at our bacteria through microscopes, they would be scared and go home.
All ghosts do is ruin your furniture and occasionally haunt you to death. Also, you can't do a ghost but a ghost can do you.
So with the science out of the way.....be afraid, be very afraid.