I doubt it's hair algae, since hair algae looks like hair and not snowflake-tipped seaweed.
Does it look like it has little tiny grapes on the end? If so, that's caulerpa. It doesn't sound like chaeto, which looks like a small ivy plant almost.
Is it overgrowing or overtaking your tank?
If you do determine that it's caulerpa, then you need to be careful with it, because caulerpa can sometimes go sexual, meaning that it releases chemicals in the water as a defensive measure for the purpose of multiplying itself. People that use caulerpa in their refugiums leave the refugium lighting on much longer - some of them for 24 hours a day - so that the caulerpa doesn't go sexual.