The thing which is green when you see it in the morning, black if you see it at noon, blue if you see it in the evening, and white if you see it at night is algae.
To be precise, this refers to blue-green algae, which are known to change color depending on the period of the day. These types of algae are known as cyanobacteria, and they can be harmful to the water they are found in, as well as to living beings.
