Put a baking tray filled with water into oven and preheat to 300°F/150°C. Wrap the outside (bottom and sides) of a 10-in (25 cm) loose base round cake pan with two layers of aluminum foil (to prevent any leaking). Beat cream cheese and ricotta cheese until combined, then add in sweetener and beat until smooth. Add in green tea powder and mix until well blended. Combine egg yolks and 1 egg in a small bowl, then add in gradually to the cheese mixture and beat until the batter is smooth. Add in Greek yogurt and mix until well combined. Set aside. In another clean bowl, whisk egg whites till foamy, then add in cream of tartar and continue to beat until stiff peaks form. Lightly fold the beaten egg white into the cheese mixture until just blended. Pour the batter into the prepared cake pan and bake in a water bath for about 1 hour 40 minutes. Remove from oven and set aside to cool completely. Keep refrigerated for a few hours to chill the cake completely. Slice the cheesecake with a hot knife and serve chilled.