What is this slimy black gunk that came out of my windshield washer reservoir?

38,000 miles mostly spent in a garage. I’ve had it for a year.

Slimy black stuff that came out of my washer reservoir. That was a lot more than what is shown.

 

Biofilm, a community of microorganisms (such as bacteria, fungi) that attach to a surface and are surrounded by a self-produced, slimy layer and later dry up over time.

 

I think the washer fluid with ALCOHOL (Winter fluid) can be used year round to prevent this.
The Winter fluid seems harder to find in the Summer. Maybe it evaporates faster with the alcohol?

 

Even after removing & cleaning it out, it still had slime and biofilm stuck on the inside.

I used some Puriclean Water Purifier for Camping & Caravans and left it overnight.
Rinsed it out a couple of times and all seems well now.

If anyone has the original problem, don't remove the reservoir. Just drain it out and treat it with the Puriclean.