As far as I can tell, "Grecian" and "Greek" both mean "of or pertaining to Greece." Is there any difference at all between them?
In Modern English, Greek is the usual adjective meaning of or pertaining to Greece.
Grecian is an earlier construction, with an adjective-forming -an suffix (American, Norwegian, Virginian), which is now pretty much relegated to stylistic and fixed phrase duty. It's common in the following expressions, among others:
But there is no Grecian restaurant, Grecian wine, or Grecian language.