as do a lot of German monarchs which leads me to believe that it may be the insignia of the german protestant branch of the order of st john. For example Prince Leopold IV of Lippe
"All members of the Order may also wear a plain, Maltese cross as a star or 'breast badge'. Most such stars are of plain linen, though enamelled stars in either silver or silver gilt, of about 5.5cm in diameter, also are worn in formal evening attire"