The lack of oxygen is the worry, not moisture per se. If it is real moss, I don't think it is a problem.
I have maples growing on an 18 inch porcelain floor tile. The root populated substrate is covered with growing green moss. This is the third year I've had the group of them in this composition. My conclusion is that moss admits/transmits enough air for the tree roots to thrive.
My caveat (above) comes from experience with a species that looks like moss, but presents itself as little green asterisks. On its own it develops into pretty green clouds. I cultured it for a season and then realized it was the cause of two trees dying the following spring. I don't have a species i.d. for it and suspect it isn't a true moss as it seems to have roots and those roots form a dense hairy mat that, evidently, not enough air can get through. Anyway, I now get rid of this stuff anytime I see it - even plucking individuals with tweezers from my 'carpets' of real moss.
On the other hand, it should not be difficult to simply rub the moss from the exposed roots with a little gentle stroking with a toothbrush (and a plucking bits here and there with tweezers/forceps) if your design is emphasizing exposed roots.