What proved to Greg that the grout monster was real? Diary of a wimpy kid book 13

Because of his mother told so and he saw black hair in the bathroom

Explanation:

One day, Susan jokingly tease greg and told him that there is a monster called grout in the bathroom. She said to him that the mosnter is really nasty.

Greg saw a glimpse of gritty hair in the tiles and convinced himself that it's part of grout's body. He imagined grout as a scary hairy creatures that can freely move even to the smallest space.