blessOTMA, you have a very good imagination;).
I think (sure) that this man with Olga and Vyrubova on photo is just one of the HIM Cossack Guard's Convoy officers (rank "Sotnik", US - Lieutenant). Nothing more. Of course he can have some resemble to Christopher of Greece, but it's only a good coincidence. Moreover, the man on photo has a typical southern dark appearance (Georgian or more likely, Azerbaijanian). Nothing from the Greek royalty;)
Btw, as I can see, photo itself is taken on balcony of Vyrubova house in Tsarskoe Selo. So, it's 100% officer of HIM Guard's Cossack Convoy (and not for example Terek Cossack), which was dislocated in Tsarskoe Selo.