Complex sentence: Under the Ottoman Empire's Millet system, Greeks were in the "Rum Millet" (Millet-i Rum), and also in today's Turkey "Rum" are the Turkish citizens of Greek ethnicity.
Split into simpler sentences: Under the Ottoman Empire's Millet system, Greeks were in the "Rum Millet" (Millet-i Rum). In today's Turkey, "Rum" are the Turkish citizens of Greek ethnicity.