Growing mushrooms 

Used mostly in anime and manga, the term growing mushrooms refers to when a character has become immensely depressed or saddened. The character is often shown to literally grow mushrooms on their body, signifying that their soul is, for the time being, in a dark and dank place. (Mostly occurs with more dramatic and over the top characters who can't stop crying every 2 seconds 😒)

Senpai: (crying because the girl of his dreams thinks he's dramatic and egocentric)
Girl: "Senpai, quit growing mushrooms around me; the smell is absolutely noxious!"

Growing mushrooms by zoy70 June 3, 2018

growing mushrooms 

Sitting for a long while

My little brother hasn't been doing his chores, he's just hanging around the house , growing mushrooms.

growing mushrooms by Little dapper August 10, 2017

Growing a Mushroom 

getting an erection, usually used when a surprise boner begins to form

As Jim's pants started to get tight he stated, "damn i'm growing a mushroom".

Growing a Mushroom by benny_64 March 17, 2009