They might find it really creepy and mysterious. It could have a big impact on their imagination and maybe even their fears.
Girls who believe in the Slenderman story could have different reactions. Some might become obsessed, constantly thinking about it and sharing it with friends. Others might keep it to themselves and just enjoy the thrill privately. It really depends on the individual and their personality.
For some girls, believing in the Slenderman story could be a way to escape reality or explore the unknown. They might get really drawn into the fantasy elements and the sense of adventure it offers. But for others, it could just be a passing interest or something they think is more fictional than real.