Have you ever been to a florist shop. They have display cases where they keep their flower arrangements fresh. It does it by keeping the arrangements cool.
If you want to grow moss indoors then you would need something similar: good light, humidity, and coolness are essential.

Here is a link that might be useful: Example at a retailer site

Thanks, I thought moss needed darkness? didn't know about coolness.

Some moss do better with a higher amount of shade than does others. Many of the prostrate moss species will tend to prefer partial shade, or about 2 hours of direct sunlight-preferably in early AM or late afternoon, than do many of the erect species of mosses, such as Polytrichum which can do quite well with 3-4 or 5 hours of direct sunlight-even at midday if the temperatures are cool. Most mosses prefer coolness to heat any time of the day or night. While some mosses, such as desert species and some of the very small erect species, such as Bryum, endure hot spots such survive by going dormnant until cooler and dammper weather occurs.

Here is a link that might be useful: Article on desert mosses

If you are going to attempt to grow moss on a concrete structure you should select species that prefer alkaline conditions. These could include ones that you find growing on concrete or other calcareous (e.g. limestone/dolomite) substrates. Same would apply to wood, although the type and age of the wood could be important. A mist system really helps but may not be practical indoors unless you have a greenhouse.

I'm not sure if this is normal with moss and buttermilk methods, but when I tried to get some to grow in that manner, I ended up with furry grey mold additionally, when outside.

I haven't tried inside yet, but had a few various thoughts crop up recently. Possibly trying to create cheesecloth 'sheets' for it to grow on, then wrapping sculptures in it after the moss has taken off well?