I don't know if this will help or not. I'm no expert on growing moss on rocks in water gardens but I have an idea or two that might help.
First of all I think that possibly the type of rock might be very important. I have limestone boulders in the woods around my hayfield that the old timers cleared out of the field 100 or more years ago. These rocks have no moss growing on them, ever. no mater where I find one, north slope, damp conditions, rocks all around covered in moss.
They are easy to spot in the woods because they're the bare rocks with no moss on them.
At the other extreme I was taking the scenic route through the woods and saw this rock that was so completely and beautifully covered with this tight dense short moss of only one species that I had to stop and caress it and photograph it. It almost didn't have a square in. of uncovered surface. No other rocks in the area had moss like this. Lots of moss of different types on other rocks but never completely covered. There is definitely something special about this rock as far as that particular species of moss is concerned. I believe it's a sandstone rock from up the hill that came down in a land slide and rolled down the hill.
This makes me think that different species of moss like different kinds of rocks to grow on. This leads me to imagine landscaping a waterfall or rocky area with different kinds of rocks that will grow certain mosses best, mixed with rocks that won't grow moss at all. The mix of rocks each with it's best moss for it should make a self maintaining segregated mix of mosses and un-mossed, is that a word, rocks.
Another idea I have is about starting moss on rocks, either from spores or transplanting. I have read about growing moss in containers of potting mix buy sprinkling spores on the damp surface covered with plastic in a non sunny window. I can imagine, especially on a rough rock like sandstone that you could take some potting mix and smear a thin layer into the pores of the stone, just a few millimeters thick, wet or dry. I'm thinking wet mix on a damp rock might be best. I think that this thin layer of soil between the moss and the rock, just like the soil that moss creates and holds between the rock and itself in nature will help a lot in getting newly transplanted moss or spores to attach to the rock and grow well.
Another idea is to fertilize the moss by sprinkling some very fine compost or soil into established moss on rocks and brushing it into the moss with your hand and then watering it in. In nature things are always falling into moss, building up it's soil layer and fertilizing it. This soil layer evens out wet and dry periods and prevents the moss from dying to much in very dry times.
Dry times are helpful for moss to maintain its domination of it's rock or log. Rocks especially are hostile to seedlings simply because there is no soil layer to hold water. practically any seed will sprout and try to grow on a rock if you keep a mister on it all of the time. but if you let that sprout dry out for 5 minuets it will probably die. Moss is actually quite resistant to dry conditions, as long as it's not TO dry for TO long. I am quite sure that the moss covered rock that I described was very dry during the drought of 07, 88, 99, and on and on back in time. I believe the moss on that rock has been there for more than 100 years because there are at least 200 year old trees growing over it and I think it would take a hot sun during a drought to kill or prevent it from growing. The rock of course has been there for thousands of years, and maybe the moss with it. My idea is that a tree seedling growing in the moss on top of this rock, that is maybe 10 years old and still is only 8 or 10 in. high, with equally long roots would not survive one of these droughts, that the moss apparently survived, along with all of the other moss in the area that lives on rocks. This keeps the moss weeded, so less competition.
Of course various mosses have various ability to withstand dryness I am sure there are mosses in rain forests that cant be dry at all. And I know that pillow moss is the most dry resistant moss around here, and it needs a dry hot rocky windy ridge to grow on.
Here's a pic of that beautiful green rock I was talking about. The turtle is just cute!
