Itās actually a technical term in language theory ā a term that cannot be produced by the language, but is still considered part of the āuniverseā (i.e., the set of all terms) is called āgarbageā or ājunkā.
Since I canāt find a source online to verify this claim, this might just have been the case in my coursesā¦
"Invalid" or "unparseable" are more understandable descriptors in normal language. I don't think I ever heard of garbage/junk being used for that in language theory but it may be domain specific usage.