I've tried it myself, it's possible, but don't expect results similar to those in the article. Getting it to grow on a vertical wall is very hard, it won't hold the water and unless you have windy rain it won't really get enough water in the beginning. Because before it's become nice moss, it's more like dry grass on a wall. I've experimented with adding super absorbent polymers to hold the water ("water beads for plants" or something like that). With that, I got better results. But mostly on angled/horizontal surfaces. For some reason, my country doesn't sell that kind of stuff, so I had to cut a bunch of diapers open.
These people (https://www.youtube.com/watch?v=GBsIljmgm7o) seem to show it working with diaper-stuff like you mentioned, though many people are crying "fake" about their video.
Man vs. Pin tried (https://www.youtube.com/watch?v=yMfwen84Wmo) and didn't have much success, though in the end a neighbor washed it away, and it looks like he was putting it on a sunny wall in a warm climate.
It'd be neat to figure out, but doing something like a vertical garden where you start out flat, grow the moss, then hang it on a wall seems more realistic to get anything like the Anna Garforth pieces everyone seems to post along with the recipes.
Guides like this one often include the work of Anna Garforth (http://www.annagarforth.co.uk/) but she uses a different method. She grows moss and cuts it into typography, then adheres it to the wall. I have yet to see a real example of grown moss graffiti.
I wonder whether a business would be required to clean up moss graffiti like they're required to remove painted graffiti. After all, moss does grow everywhere naturally in this city. I even have some on my balcony on 20th floor.
It gets hosed off shortly after.
Cool project, awful title. "Most Illegal DIY Project Ever?" Seriously, I thought it was going to be a home-made suitcase nuke.
Edit: What the guidelines say has always been the policy. For example, that's why we changed the linkbait title to "Moss Graffiti".
I think the reason for the misconception is mostly sample bias. People don't see most of the cases that get silently fixed and/or don't realize that we fixed them. Only the ones that seem wrong stand out.
"The only way we can tell if a newly created title is accurate is to read the article, and we're not about to read every article submitted to HN. The only option is to revert to the original title, which is at least what the author intended." (https://news.ycombinator.com/item?id=6572466)
After that, it seemed like people kind of gave up the fight and just started leaving the original title more often.
(I don't mean to argue with you here. I gladly accept that it is not the way that post seemed to indicate. I'm just trying to identify the source of the disconnect.)
The only thing we changed about titles since PG handed off moderation duty is to emphasize that it's ok to take other language from the article, such as a subtitle, a photo caption, or a representative sentence, if the formal title doesn't represent the article accurately. It's nearly always possible to do this, so it strikes the right balance between wanting informative titles and not letting submitters editorialize.
But even that is barely a change. PG mentioned subtitles in the post you quoted.
The only challenge is getting fissionable materials.
An alternate form of this is cleaning the sides of dirty buildings to leave messages/art.