It's a common mistake thanks to people learning in school that the "primary colors" are red, yellow and blue but the explanation is mostly right otherwise: you see cyan because it's a mix of blue and green, i.e. what you get when you subtract red from white light.

Of course human visual perception is a bit more complex than that but the primary mistake here is mixing up RGB and RYB. Quirks of color perception then result in referring to the resulting color as "green" when it clearly contains blue.