How to trim moss

[plantz]

Hi,

I have taiwan moss growing thicky on both my moss wall and a piece of large driftwood. The moss wall is done by folding black plastic mesh into half to hold the moss in between. For the driftwood, strong thread is used to secure the moss tightly on the driftwood.

How would I know if its time for a trimming. I don't mind having it thick, however heard that the bottom layer will not be able to receive light, thus rots off. How do I go about trimming?

Thanks.