Freshly mixed saltwater...
Seriously though, any chemical that is added to the tank would just be a bandaid. Something is feeding the hair algae, eliminate the food source, and the algae will go away. This may be caused by overfeeding, phosphates in your sour source water, the photoperiod being too long, etc.
If you post all of your parameters, including feeding schedule, what you feed, tank inhabitants, tank age, lighting schedule, water change schedule, info on the setup, etc, we can better help you identify and fix the problem.