I think the jury is still out on it's effectiveness as an algae control. Some folks swear by it, others swear at it. A few studies suggest that alkalinity plays a huge role in it's effectiveness. Purdue tested it, and achieved good results in some areas, not so good in others. Definitely not understood fully. The general consensus is that it will not kill existing algae, but may prevent future growth, AND, there are many varieties of filamentous algae (green slime), and some may be more susceptible than others.
I use Cutrine plus for control of FA.