Airline can change colour with use over time. Some people report it turning a cloudy colour whilst others report blue or even green! This is nothing to worry about; it is perfectly normal and is simply a result of temperate fluctuations as well as oxidisation of the inner tube.

This will cause no harm to the fish and will not negatively affect the use of this tube within your aquarium. It can sometimes be algae growing on the exterior of the tube, giving it that green appearance.