Serratia marcescens is causing the pink color.
It`s a gram-negative bacteria that is considered to be a human pathogen,
though it is commonly found in your digestive tract,
just like e.coli...
in low enough numbers to not harm you.
The reason it grows in your shower is because it is a moist environment and the bacteria are nourished by the residues of the phosphorus-rich shampoos and fatty conditioners or soaps you are probably using.
It is not dangerous,
though it has been known to cause infection in open wounds (so be careful if you have cuts on your feet),
and a number of types of eye infections.
It`s difficult to remove the colony of bacteria entirely once it has established itself in your bathtub,
but disinfectant with BLEACH should eliminate the bacteria for some time...
stopping any dripping faucets will also help,
as will routine cleaning of the shower or tub to remove any soap residues that the bacteria normally feed on.