The Grand Duke is wearing the collar of the Order of the Star of Romania, the republic's highest decoration, and I think you already knew his star is of the Luxembourgeois Order of the Golden Lion. On the Wikipedia page (link below), the highest class of this order is listed as "Grand Cross with Collar", but I looked it up in Burke's World Orders of Knighthood and Merit, and there top grade is listed simply as "Collar", which possibly means there is no sash or star in this grade, and that might explain why he wore his own star with it...