It does not. It sterilizes free floating bacteria and parasites, depending on the contact time. If its already on a surface, they are of no help. So it will kill off free floating algae, already in the water column, not on any surface.
Will not have much if any effect on hair algae which grows on substrate. UV sterilizers usually only work with algae that grow in the water column--usually only found in freshwater though on rare occasion happens in saltwater. UV for saltwater is used primarily for disease control by killing pathogens in the water. If you have hair algae you should keep an eye on phosphates and use control methods such as Granular Ferric Oxide and water changes.