Guerilla Gardening means all kinds of things, from beautifying neglected public spaces, to growing food in those spaces, to harvesting unused food on public land, to making political statements with shrubbery.
Years ago some friends and I had fantasized about delivering a public message to a homophobic religious group who happened to have a large, freeway-facing lawn. We planned to write our message in vinegar, letting the grass die over a number of sunny days. (It's just a less toxic version of leaving a message in glass etch on a shop window.) This project never happened, alas, I still look at horticultural messaging with longing. A teen friend showed me up by mowing "fuck you" into her parent's lawn when pressed to do chores. They thought their lovely daughter was so brilliant they had a hard time being angry.
Sometimes, the plants speak for themselves. Other times you need to deliver a botani-gram. Here are one|two|three! methods for moss graffiti that are very enticing. I'd stick with beer or soy yogurt instead of buttermilk to keep it vegan.