Inside the dog water bowl is emptied everyday (thanks to the water buffalo I call Buddy ) and I clean it a few times a month.
It never gets slimy.

But I have a 5 gallon pail outside that I keep near the garden hose and I dump it and refill it once in a while and I have to make sure to remember to wash it out once in a while. Only once have I seen the bottom of it getting green and slimy. I felt so bad.