The same thing happened to me some years ago. I think this idea comes from some moss authorities (G. Shenk) who recommend using Roundup to clear an area BEFORE planting moss, it is good because you can plant as soon as it dries. I also suspect that some moss varieties are more vunerable to the spray (especially the 'leafy' kinds) and if they are wet or dry at the time (if dry they will absorb the spray?)
The good news is that the moss will come back faster than the weeds/grass so be patient!

My moss turned brown initially and then came back.

information from the URL that tenbsmith posted.

"Glyphosate

One of the most common herbicides in use today is glyphosate (for example in Round-Up). This herbicide is often not effective against mosses (Woodfill 1999; Roberts and Ziegenhagen 1999; Newmaster et al. 1999) but does kill mosses in other cases (Newmaster et al. 1999). This chemical is absorbed through the leaves, becomes tightly bound to the soil, and is degraded by microbes. The failure of many common herbicides against mosses can be seen dramatically in some christmas tree plantations and other perennial crops where competing higher plants have been killed by herbicides, leaving a green carpet of mosses and other bryophytes.

---> The situations in which glyphosate does or does not kill mosses remain unclear I sprayed small area frist. All the moss did great. Then syrayed biger area and all the moss did great. Yesterday I sprayed a big area (use 10 gallons). In this area the moss was already somewhat brown.

Here is a link that might be useful: Chemical Control of Moss in Lawns

I don't remember exactly how long it took for the moss to green up again. I think it had to be a couple of months on average. In the meantime, give it some extra water if you don't get much rain.

We used RU on grass/moss to start a new bed. This was 6wks or more ago. Moss slowly browned and has not greened up again. Its still there and may be alive but looks really bad. So if you WANT your moss, I wouldn't spray RU since even if it doesn't die it looks pretty bad and would be slow to recover.

I'm testing a new theory of mine - to use vinegar instead of Roundup to kill grass and weeds in mossy areas. I figure that the acidity of the vinegar will not bother the moss, but will be effective against many weeds. In particular, I want to control wild violets. All the other weeds I can remove, the violets are a huge pain to get up and they are all over the place!

I'll let you all know how it turns out.

Hey shadylady- Did the vinegar work on the moss for you?

I have found that by using the lowest rate of Glyphosate that allows control on grass and weeds in your moss area has minimal effect on the moss. It is important to be mindful of an accurate application for best results As with any pesticide, the most important aspect of application is the "rate" or amount of concentrate for a given area. The amount of water it is mixed in is less important. In other words, as an example, if the label says to use 1oz. of any pesticide per 1-3 gallons of water per 1000 s.f. of area to be treated, whether you use 1,2 or 3 gallons of water, apply only 1oz. of pesticide per per 1000s.f. treating less area would effectively be increasing concentration and visa/versa for covering more than 1000 s.f. By making an accurate application, you will accomplish a more effective result.