hiring a real ghost would be easiest, but I've heard they are very expensive, and that the union is horrible...
If you wanted dimension I would suggest using a hazer or a smoke machine, depending on the look your going for. At first I thought you meant a real
ghost light, like the light you leave on the stage once everyone leaves. I would have suggested a guy in all black pulling the light. If the light just needed to go across the stage then you could possibly have someone pulling a cart with some black light target on it across the stage, but if you want it to
go up and down stairs, or really do anything but go in a straight line, that would be difficult.
I think the best option would be to use your spot light, along with a hazer. If you wanted a
gobo'ed look, then I would suggest setting up a S4 with a gobo near a rail that someone could operate like a spot light--possibly with a gobo rotator in it as well.
I don't know what your application is, and if this is a high school show or something larger, but ML's are expensive and take time and energy to program and set-up the way you want to. If you have the time and money, then a ML would be cool, since you have all of the color's and gobo's and can rotate all of the gobo's and all of that stuff. However, that is difficult to set up.
I think that a spot light with a hazer would be best.[/i]