I actually keep a squirt bottle of bleach/water mix right at my sink--when I open, I spray inside and on the base and scrub quickly with a long brush--that mold can grow really quickly, especially in warmer temps! I do it every day in the summer and warm weather and every other day in fall and winter!
I've never seen mold in our waterers. Just random things that the birds might scratch into the water.
Some people put a little apple cider vinegar in the water to prevent algae from growing. I've never had algae grow in our waterer either, though. I have however started using vinegar because it is supposed to be healthy for the birds.
We only rinse our waterers out with fresh water. Every couple of days when we refill them.
When they were chicks I washed them. Now I change the water twice a day so nothing sits in them very long. I make sure there is fresh water every night so I don't have to change it first thing in the morning 6:30am