The pink stuff that is on your shower head is a type of mold.
It's called pink mold which grows on shower heads and shower walls sometimes as well as bathtubs.
The pink mold gets it's pink color because of a pigment (prodigiosin) produced by Serratia marcescens under the right growing conditions.
The pink mold is caused by bacteria that is growing in your shower.
You should clean your shower and shower head often to prevent the pink mold from growing.
It's recommended to clean the shower and shower head using some vinegar which kills the mold off.
Clean the shower head with some diluted bleach every few days or so to kill the bacteria which will prevent the pink mold.
If you have pink mold be sure to clean it off because it could potentially make you sick and it's also disgusting to look at.