While praying asr prayer I felt my hayz however it did not reach my clothes and after I finished my prayer and checked for confirmation, it was the very starting and it came out with lot of white fluid so is my asr prayer valid or I have to do qaza?

As soon as the first drop off hayz comes out, it invalidates the prayer. The criteria for start of hayz and subsequently making the namaz invalid/void is the coming out of the blood.