There are two methods described online for making this type of live wall art.
#1 The milkshake technique - Collect a few clumps of mosses. Grind up the mosses in a blender with buttermilk, beer, or some other liquid to help the moss stick to the wall. Then paint the mossy solution onto the wall. Water the mossy area to help the mosses to grow.
#2 Collect patches of mosses. Trim and arrange them into the desired design. Then paste/attach them onto the wall.
The problem with many of the descriptions online is that method #1 is outlined and then art made using method #2 is displayed as the result! There are many before photos and videos for the milkshake technique (here, here, and here), but hardly any after photos. This is the best one I could find. Why do very few people show the results of the milkshake technique (method #1)? Well, mosses are not fast growing plants. After painting mosses onto a wall they need to be kept constantly moist and will take a long time to grow. I think that many people try out this technique and few plants grow.
Why does this technique seem to fail so often? In theory this technique could work. All moss cells are totipotent, meaning each individual moss cell can regrow an entire moss plant. However, moss species are often very specific to where they grow. Some species grow only on soil, others on trees, and still others on rocks. The success rate will probably be low if mosses that usually grow on soil or wood are painted onto concrete or brick walls. Another reason they may not grow is that the surface is not moist enough.
The vast majority of the moss graffiti images online were not made using the milkshake technique. The lush mats of mosses in the shape of a creature or phrase with crisp edges were instead made from fully grown mosses. Very few sites describe the process wherein they harvest mosses, cut them into those amazing shapes, and then use a paste to attach them to the wall. Most mosses used for horticultural or ornamental purposes are collected from the wild. The mosses used in these pieces may be able to stay alive or continue to grow for a while, but I wonder how long they can stay alive without constant watering and maintenance. My guess would be that they can stay alive for a bit, but in the end they would eventually die.
These urban displays of mosses are beautiful, but I think that it is important to consider the cost of this beauty. Where was the beautiful moss taken from to put onto this wall?
I think it is a fun activity to move mosses around the yard on a local scale to landscape with mosses. I am also in favor of purchasing mosses from a grower that produces them sustainably or rescues them from development sites, like Moss'in Annie. However, I imagine that many of the mosses used for these art/graffiti installations were not sourced in an eco-friendly way, but were instead pillaged from the wild. The strip mining collection of mosses is a big business with much of the collection in the United States happening in the Pacific Northwest and in the Appalachian Mountains. Personally and professionally I am opposed to this type of collecting. Many of these mosses are long lived plants that are growing in old climax communities. They could easily be 10 to 50 years old and it could take at least that long for them to regrow. Wild collection on a massive scale is just not sustainable industry.
Consider a hunt for urban mosses that are hiding in plain sight. Any moist place is a great location for mosses to grow. In the cracks or at the edges of the sidewalk. Beneath a dripping window air conditioner. Around the base of a tree trunk. Mosses are writing their own graffiti and they are adding a bit of green to the man-made world that surrounds us.
An alternative to adding mosses to walls is to remove some of the mosses that are already present to create a design. I thought this one was a pretty amazing display of that technique.