Any idea what this is?

[shimmer]

It's almost feathery. My first thought was GHA, but it's brown.

 

[Triggreef]

gha... it can look different sometimes based on its health, just like our corals do.

 

[vetteguy53081]

It's almost feathery. My first thought was GHA, but it's brown.

Not GHA but an algae known as Bryopsis.
This is one of the more challenging of algae due to need of its complete removal. Removal is best accomplished by taking the rock out of tank and placing in a container of tank water. Then you will want to pull as much as you can by hand and discard. With a dental pick or small crochet needle, pull the roots off the rock. YOU MUST GET EVERY ROOT OR IT WILL SIMPLY RETURN !!
After you are done return rock to tank and reduce white light intensity and even hours of white light and add the following cleaners which will help with control. These guys will consume bryopsis but not as fast as bryopsis can grow - They will never keep up.
Pin cushion urchin, Chiton snails, pitho crabs, and larger astrea snails

This procedure is not as bad or time consuming as it seems. Assure phosphate levels do not become elevated which helps feed this algae

harbor Freight: