The term you're looking for is "cultural racism" (or sometimes "neo-racism" or "modern racism"). As an aside, I dislike all those variants, and would prefer something like "culturism" or the perfectly applicable terms " nationalism, "xenophobia", and "mixophobia" (or, to mention a common highly specific form, "islamophobia").
On the other hand, "cultural racism" seems like an appropriate term when a highly diverse (nationally, linguistically, ethnically, culturally, etc.) group such as "Muslims" gets racialized into a prejudicial stereotype.
Anyway, it is basically essentialism applied to culture instead of race: the ideas and arguments of both are deeply tangled, with cultural arguments often used as proxies for racial ones where the latter would be rejected out of hand. For example you see it pop up in demands that immigrants assimilate, or claims that immigrants don't (or won't) assimilate used as justification for discrimination. It can also appear as a mixophobic demand for cultural segregation. Or, as here, cultural differences apparently used as a veiled form of nationalism.