I think you are overlooking the obvious answer. Ghosts that can manifest themselves physically (i.e affect objects in the physical plane as opposed to just appearing and disappearing) must coexist on at least some level with material objects. In this case it's the ectoplasm.

So, in the case of Peter being slimed, the non-corporeal ghost zooms toward Peter, manifests itself physically and attacks, passing through Peter in the process and knocking him over. ONly the non-physical aspect of the ghost passes through, which would leave Peter unharmed. The ectoplasm, though, since it only exists in the physical plane, cannot pass through, and thus covers Peter (and the wall when Ray chased him).

A ghost that manifest within a person would likely just possess them, and thus, not create any ectoplasm as a by-product.

At least that's what THIS geek thinks.

I think Phineas' post makes a lot of sense. Of course, it's kind of subjective, considering nobody really knows the science of ghosts, which may remain the case until scientists start giving the matter some credibility instead of just assuming such things are dellusions or whatever...

I agree with Phineas. The energy that makes up the ghost may be able to pass through objects or people but the ectoplasm itself would be sloughed off on the outside of that object or person.

When we slimer force/phase himself through the wall after being shot at by Ray, we only see the slime on the wall. If ghosts, atleast the type of ghost he is, did drag ectoplasm with them when they go through things we would have seen some change in consistancy. As this is purely speculation is it not possible that the slime is either produced by the base spirit of a ghost as a thin membrane so that they can interact with the physical world. Ghost, at their most basic, are driven by emotion. So the production of ectoplasm is stimulated as needed, in Slimers case it is so he can interact with food and such and try and fill his stomach but instead the food slimes down, taking some of his ectoplasm with it.

The amount Peter was covered with when attacked prooves my idea to a degree as it was far more than when he was attacked by Ray. He was scared, angry and ready to fight back, having built up ectoplasm so he would cause more damage, kind of like a cat bringing out its claws. He would of recognised the basic colours and shape of Peter being the same as Rays and proceeded to get his revenge, after which his primary driving force, to eat, kicked back in and he proceeded to the biggest food source, the party food filled Alhambra Ballroom. When met with the sight of 3 of what he assumed was 1, he completely sheds his ectoplasmic membrane in an attempt to save himself from further attack. If you watch the scene from inside the Alhambra Ballroom, you'll see his basic shape shifts and distorts as he tries to move and hide quickly. His tiredness just before he is captured is most likely from the Ghostbusters proton blasts and from how many times he had to shed his ectoplasmic membrane.

I've typed too much, sorry for that and possible spelling mistakes.

Very good post.