It looks like Xuanya scratching on Hoya's back.

Funny because it looks funny, right? hahaha

In fact, hoya has a tattoo with infinite symbols on her back, which is why Hyun Ya opened her coat.

But if you only look closely, you will find that Hyun Ya's last hand (the hand placed on hoya's back) has an infinite tattoo.

Give me a reward for such a serious answer, o_o, thank you for stealing ~