Yikes. We had a watercooler catch basin dubbed ‘The Love Canal’ (back when that would have been a racy double entendre.) Someone liked to dump the dregs of their coffee regular in there and the microbes were off to the races.
Even the inside of my humidifier when I went too long without cleaning it this summer (due to far less use) never looked that bad.
Oddly, that was the only time I’ve seen green (algae I assume) in the unit, and it was only in the reservoir, not the base. Usually all I see is the orange/pinkish stuff.