David Peters wrote:

After several months of usage, the hose of a push-on shower spray
(http://www.diynot.com/img/ht/plp004.jpg) can get black slime and
mold(mould) on the inside.

However the same push-on shower device will NOT get any slime on the
rubber-like cups which push onto the taps. Nor will it get any slime
on the plastic shower head.

Why does only the main hose get the slime?

Slime is produced by a slime bacteria. Many metals, such as silver, zinc
and copper are quite good bacteriacides. So if the fitting has any of
these metals in it, slime bacteria will have difficulty growing on it.

Marshall