I purchased some xmas moss online in early November that did the same thing. Mine came in index card sized sheets. Half of it was brown when I received it. I used what I could of the greener portions, although it started browning a week or so after. I was told by the seller that browning was to be expected... that it would recover once established in a submerged environment. It did, however, the decaying bits caused a massive green algae outbreak. Nothing worse than fighting rhizoclonium/GHA in some dense moss. I had to go to extreme measures to knock it back but I'm slowly seeing results. I will most likely rip it out and treat it outside the tank in a bucket of water somewhere in a dark corner for a week or so. I plan to do a little re-scape very soon. The good news is, it is very hardy. I have some extra xmas moss from the same batch attached to some lava rocks that have sat in a cold bowl of water in a window seal for over 3 months. I change the water like twice week. Just straight tap water. It's doing really well.