Diagnosis
That pink slime is Serratia marcescens, a common bacteria that thrives in damp, dark places. The reservoir's design has seams and the pressure dial assembly that trap standing water and promote growth.
How to Fix It
You need to disassemble and soak it. 1) Remove the reservoir. 2) Turn the pressure dial on the front to the highest setting (10). 3) Look at the back of the dial—there's a small tab. Use a small flathead screwdriver to gently pry the dial knob straight off. This reveals the inner stem. 4) Now you can fill the reservoir with a mix of 1 part white vinegar to 3 parts hot water. Add a teaspoon of denture cleaner tablets for extra effect. 5) Let it soak for at least an hour, agitating it every so often. 6) Use a soft bottle brush to scrub the inside seams and the now-exposed dial stem. 7) Rinse thoroughly. Re-attach the dial by pushing it back on.
If That Didn't Work
For stubborn stains, try a soak with diluted hydrogen peroxide (3% solution). Do not use bleach, as it can degrade the plastic and leave a permanent smell.
Heads Up
Prevent this by never storing water in the reservoir. After each use, empty it, leave the lid open, and store it separately from the base upside down on a towel so it air-dries completely inside.