Oh, I see... How do you keep the moss wet then?
It doesn't have to be aquatic moss. Terrestrial moss would work just fine (even though technically most moss grow terrestrially naturally, some are just tolerant enough to be grown fully submersed). I think the humidity and the splashing would be enough moisture for something like fissidens if it is given enough time to adapt to dryer conditions.
Or I guess you could spray it every day. *shrug*
Another thing I have seen people with frog terrariums do is create foam backgrounds and cover it with silicone, then press dried sphagnum moss or coco husk all over the wet silicone.
For example: http://bigalsonline.blog.com/2011/08...um-background/