COOK MACARONI IN BOILING WATER FOR 8-10 MINUTES (OR PER PACKAGE INSTRUCTIONS). DRAIN AND RINSE AND SET ASIDE IN LARGE BOWL.
CUT THE HARD BOILED EGGS IN HALF AND REMOVE THE YOLKS, AND PLACE THE YOLKS IN A SMALL BOWL. CHOP THE WHITES AND ADD TO THE PASTA BOWL.
MASH THE EGG YOLKS WITH A FORK. STIR IN GREEK YOGURT, MAYONNAISE AND MUSTARD.
ADD THE CHOPPED ONION AND CELERY TO THE PASTA BOWL, AND STIR IN EGG YOLK MIXTURE UNTIL COMBINED.
TOP WITH FRESH CHIVES, PAPRIKA AND SALT AS DESIRED. CAN BE EATEN IMMEDIATELY, BUT BEST SERVED AFTER BEING CHILLED IN REFRIGERATOR FOR AT LEAST 1 HOUR.