Why does empty part of water container turn yellow?

Question

I fill drinking water from the water vending machines in 5 gallon containers. Lately the top part of the container, where there is no water has turned yellow. Could this be a type of fungus/algae/bacteria or some chemical reaction? is this harmful to my health. Experts please help.

Answer 1

Could be the minerals in the water but if you wash them with a mild bleach and water mix that should help