The person hiding in the mirror was Illuso from the Hitman Team. Fugo ends up being dragged into the mirror by Illuso's Stand, Mirrorman. Fugo then summons his own Stand, Purple Smoke. Despite Giorno wanting to find Fugo, Abbacchio decides to go look for the key, since that was their mission. However, once Abbacchio finds the key, he also gets dragged into the mirror world...
That's why it's moving.
لهذا هي تتحرّك
Deshalb bewegt er sich.
Por eso se mueve.
Por eso se mueve.
qui rampe sur le sol !
Per questo si sta muovendo.
É por isso que ele está se movendo.
Поэтому он и движется.