The interesting thing (as you, no doubt, know) about Marek's is how it presents in so many different fashions. We might say there are three common variations, however the manifestation of symptoms is much more varied. The visceral tumors can develop on any organ, causing a variety of reactions (often people only think of the classic paralysis, when really it is much more "sneaky" than that) - sadly our girls mask many of those symptoms until it is almost too late. It does sound as if you may be experiencing yet another "version" - white, pasty poop being an indicator of something bacterial that might, perhaps, be just a secondary infection. (Is it accomanied by green, watery poop?)
Boosting vitamins and pushing protein are both excellent "go-tos" while you work through this.
It would be good to see a picture of both the poop and the eye.
I'm really sorry you are going through this with your sweet girl!