I think you need to look at the moss growing in your yard.
How is it growing? In shade? Is there weeds and grass growing with it? What is the kind of soil? How about the moss, what kind is it: a creeping carpet or a mat of upright stems or?
I do not encourage using any chemicals to discourage weeds on mosses but prefer to alter the way that the area the moss is growing in to be more suitable for the moss rather than the weeds, such as by daily watering, adding more shade if the moss favors shade, using the plastic cover on the soil surface in order to get the weeds and grasses to die back or just hoeing them or pulling them out.
It is impossible to have a purely mossy landscape without some weeds appearing. I even get them in the little cups that I use to grow moss in-just gotta get in and pull them out after making sure the soil is wet and the weeds can be taken out easily.

I feel terrible about the unnaturalness of this, but I take a weed wacker and just wack everything above the moss, and then water the moss if I have the time. I used to laboriously pick the weeds out of the moss one at a time, taking great care not to rip out the moss with the weeds. Wacking is much more satisfying. The weeds come back quicker this way, but I just wack 'em again.