chicken waterer algae dangerous?

I have a typical plastic chicken watererer, like on the picture below.

A greenish-brownish algae is growing on the bottom water tray and inside walls of the waterer. Is this dangerous to the chickens?

I've heard that in Australia people let algae grow inside of their rainwater tanks on purpose, so it would clean up the water of organics and some trace elements.

I don't know about the Australian's, but people with white water cisterns paint them just for this reason.  If algae in these plastic tanks isn't good for people  i would wonder about my chickens too.

For my peace of mind I would wash the inside good with vinegar (any kind, but white is cheapest), then paint the outside to limit the light getting in.  This should solve the algae problem and other sun loven bacteria as well.

It's a good idea to add some apple cider vinegar to animals water, about 1 T per gallon, as a immune system booster, with the side benefit of limiting bacteria growth in the water.