It is slowly moving up the back of almost all the fronds.
I Have a Huge 3′ Round Hanging Stag and Need to Know How Do I Stop the Brown Fungus.
Certified GKH Gardening Expert
Would you be able to send a picture? I am not quite certain it is a fungus.
But, until you can send a picture and we can positively identify it, it would be a good idea to treat the plant with a fungicide. If it is a fungus, it will help stop it. And if it is not, the fungicide will not hurt the plant.