we need more details about your tank as could be a variety of reasons and where the moss came from for starters, as difficult to kill moss..
I ask where it came from as 'in-vitro micro propagation' plants such as the Anubias brand, are vulnerable to bacterial and viral infections from the air if the tub is opened and they are not immediately planted underwater, this happened to me with weeping moss and it quickly died in the tank while everything else was flourishing..
Don't ask me why they are more vulnerable in the tub and why they are not exposed to the same nasties underwater