Dreamcatcher (2003)

Answer: Mr. Gray is not a hallucination, he is a living alien creature. Mr. Gray, and the rest of his species, are shape shifters and can also possess other intelligent life forms. He can leave Jonesy's body at will, turn into a red mist to enter another body, and turn into his normal monstrous form to kill if need be. How his species accomplishes this changing of mass is not explained.

BaconIsMyBFF

Answer: Great question. So think of it this way. Say you are infected with a small virus and it has to multiple and destroy tissue to get to its full potential i.e. the infected man in woods I think his name was Barry. Now imagine a giant full grown parasite like a giant tapeworm is ingested by an adult human, the giant tapeworm doesn't need your cells to live it's already matured, it just needs you to feed it meals from outside as it lives in your digestive track i.e. Jonsey. For your next question. So how does this thing recreate clothing? Well it doesn't, it changes the cellular gradient by turning it's molecules into what we call an airborne hypertonic solution meaning it disbonds from our cells so it's molecules can float outside the clothing and host. Then reanimate onto the outside of the host creating a cellular body to feed or kill then the cells become airborne molecules again, change their gradient back to an isotopic solution to rebond with the human cells including the brain tissue. I have a extensive medical background so I love dissecting these things. Hope this answers your question.