You could translate this as “messelium with mycelium”. “Chujnia” is a vulgar way to describe a mess, problems you are in or a product or a service a low quality. The word stems from “chuj” which means “dick” or “prick” and can be used to describe a person you don’t like as in English. By adding the “-nia” suffix, a word is built describing a high concentration of pricks or an area where pricks are at work. It is used on its own to stress that you don’t like a particular situation or what you are shown or given.
“Grzybnia” means mycelium, is not vulgar at all and used mostly by biologists and mushroom pickers. The only reason for it to build the second part of the expression is that it rhymes with the first. However, the result is extremely comical as the words don’t have anything to do with each other.