Why is Hua's face hurt by heartless water? How did you hurt it?

Because she fell in love with Bai Zi's paintings, and was splashed with water by the neon all over the sky.

The heartless pool water is used to test whether the disciples of Shushan have feelings, but Hua was moved by the paintings. As long as he meets the unfeeling water, he will be injured, scarred and disfigured, but later he will be fine. Of course, unfeeling pool water is useless to people who have no feelings.