Diagnosis
Biofilm and pink mold (Serratia marcescens) in stagnant water areas, notably the water tank interior seams and the internal reservoir.
How to Fix It
Use a baking soda paste and a soft brush for scrubbing, followed by a vinegar rinse. For the tank: make a paste, scrub all seams, rinse. For the internals: fill the tank with a 1:1 vinegar/water mix, run a full cycle, then run two cycles with fresh water to flush.
If That Didn't Work
For persistent black mold spots, a dilute bleach solution (1 tsp bleach per gallon of water) is necessary. Run it through one cycle, then thoroughly flush with multiple cycles of fresh water.
Heads Up
Never use abrasive scrubbers on the plastic water tank—it will create micro-scratches that make mold worse. Dry the tank completely before storage.