Check out these sites for an explanation and what to do:
http://www.huffingtonpost.com/2015/03/03/pink-slime-shower-gunk_n_6793586.html
https://www.ewswater.com/pink-stains-in-your-toilet-bowl-and-bathtub-heres-how-to-get-rid-of-it/
http://www.home-ec101.com/what-is-growing-in-my-shower/
