What is covering this Xenia?

[hey.al]

Over the last couple days it has been kind of closed up and looking sad. Now its little branches are completely covered. Obviously it’s not good, but how bad is it? What is it? It is only on the Xenia. It looks like there might be a little bit of green hair algae on one side. But the majority is almost brown. Anything helps! It’s my fist tank.

 

[GeoSquid]

My guess would be a sponge.