As
@moke said….I agree….I’d remove it and let the soil breathe and the nutrients available feed the tree.
Just my thought process….I wouldn't keep moss over the substrate unless the tree was headed for showing. It’s probably better in person, up front and close, but from here, in the photo, the soil looks rather thick, organic, muddy and wet….perfect for weeds. However, whatever it is, it seems to be working out well for you and the tree.