The other day, as I was adding water to my cat's bowl, I noticed a slime on the sides and bottom of the bowl. What is that stuff? Is it harmful to my cat? Could it be why he doesn't drink a lot of water?
That slimy film in your cat's water bowl is called "biofilm"; it is made up of bacteria - both good and bad "clumping" bacteria. It comes from bacteria on your cat's tongue and builds up quickly on the sides of the bowl. Cats have more bacteria in their mouths than most other species, so you may find biofilm collects faster on your cat's bowl than the dog's.
Drinking fountains are not immune to biofilm; circulating water and filters do not eliminate the collection of biofilm. Drinking fountains, with small parts and motors, are much more tedious to clean than water bowls.
If ingested, biofilm can lead to urinary or bladder infections.
Biofilm has a scent and taste that cats can detect, which may make them averse to drinking out of the bowl. Being descendants of African Wildcats, your house cat is genetically wired with a low thirst drive, and if you're feeding a majority of dry food, this could lead to mild dehydration in your cat; which can lead to renal issues later in life. (Resource: Cat Talk Radio, What To Feed Your Cat)
It is very important to clean your cat's water bowl daily. I keep mine on the counter, next to the sink so I remember to scrub and refill it daily. Run the water bowl through the dishwasher weekly. I find ceramic or glass bowls easier to clean.