Since the only saltwater tanks I have setup at the moment are nano tanks I have no overflows, but I did make myself a surface skimmer to clean the skim off. The white stuff you're seeing on the top of the water could be a number of things; but it's probably a buildup of organic waste, likely with some bacteria thrown in as well. SW tanks really should have some kind of surface skimmer or overflow box to remove the film. Not only should that junk be in your protein skimmer or filter for removal from the system, but it also blocks light from entering the water.
Same question.
Clayton