Its old coolant. Recommended coolant change is usually every 5 years. Hyundai usually have green coolant. I run Prestone in my cars no problem and with distilled water. To get a proper and full coolant change and have all the coolant released from the bock, I do a coolant change while I do the timing belt. Because then I can change the water pump, which frees all the old coolant out the block. Thats something you can look into too.