Which is world x common noun to proper noun?

The Word 'World X' is a proper noun, the name of a company, World X Productions, LLC.

A proper noun is the name of a person, place, thing, or a title. A proper noun is always capitalized.

However, the Word is a common noun when not referring to the above company. For example, 'When world X conflicts with world Y, a Science Fiction tale is born.' This use of 'world x' is a common noun.