Shockingly, the two Shingekis with dubious grasps on reality do not take this well.

Reiner freezes. Eren does what Eren does best: he marches forward to punch one of the mirrors. Thankfully, Reiner realizes what Eren is going to do and moves in time to bodily haul Eren away, preventing him from touching/punching any of the mirrors.

They retreat back up the stairs and probably have some Big Emotions at each other. Eren wants to punch the mirrors! Reiner wants to sneak back and stare at them later! This is fine!!

In any case, they eventually leave the creepy mirror tree behind and begin heading back toward the city.