What algae/seaweed is this?

It's growing from a thin brown sacky thing and its fern like.. just wondering if it's good or a pest!

any ideas??

 

BAD BAD BAD.. Hair algae.. That needs to be gone ASAP. pull the rock from tank and clean it then put it in QT if you have one just to make sure you get it all.

 

I just found a thread out on another site where a guy had bryopsis and he had found a link that which suggested boosting magnesium to 1500-1600. There were pictures that showed how it died off without ill affects to other corals.
I only glanced over it and would suggest you do your own reading but figured I would share the info..

Not sure if it is cool to post links to other sites so until told other wise I won't..

 

only certain species of byprosis respond to the mg spikes hard to tell between hair algae and byprosis in this pic though can you pull some of it out and get a good close up of it?