Hair algae or Bryopsis?
I have a similar problem, but what I thought was hair algae is actually Bryopsis. It's easy to identify - if you look closely the bryopsis looks like a fern - has feathery branches. Either way you have to lower your nitrates and phosphates to reduce the pest. I have also had some luck with lettuce slugs - Elysia crispata.
In my 12g nano cube, I removed the rock (easiset!) with the bryopsis and the slug seems to be taking care of the rest. My 29 Biocube is more difficult since some of the rocks with the bryopsis have corals attached so I reduced set up a refugium in the back reservoir, increased water changes and reduced feeding
Good luck!
