It could be an accumulation of rhizoid growth.. Mosses, liverworths and ferns do not attach with roots in substrate as other plants do.. Tho plants also develope rhizoid growth on there roots commonly called hairroots, but you don't see them after all they are in the substrate. Those rhiziods are strains of cells it uses to attach to surfaces.
The first time i noticed this was with bolbitis fern i did put in a crack of a piece of driftwood not far from the glass. And after a while it grew a greyish/white fluffy cotton like plush on it's rhizome and rootstrains. It's easily misindentified as fungus or beard algea but it isn't.
Here you see a close up of that growth on one of my bolbitis ferns.
Moss also attaches like this to hardware. Here you see a close up of rhizoid growth on a liverworth
and here on a leavemoss like javamoss or weeping etc. see the white fluffy stuff down at the rock
In your case your moss is probably not dying but just growing and branching out, reaching out for something to attach to and developing rhizoid growth on it's tips..
🙂
But as said without a close magnified look it's just a guess, it looks like that as it also looks like it could be something else.
🙂 I can't find a pic back of the dying hair algea on my moss, which looks strikingly simular, but more faul.