Is there anything that tells me how Lichens Grow?
Just from conducting a Google search using the phrase, "lichen modeling pdf", I was able to find several studies/publications that address your need(s). Though its dated 2004, this article seems extremely relevant to your application. 

Simulating and Modeling Lichen Growth


  In this paper, we present a system for modeling lichens
  and simulating their propagation and growth in a virtual
  scene.



In the article, they mention: 


Aging and Weathering
Ecosystem Simulation
Lichen Seeding and Propagation
Environmental Charateristics 
Theoretical Aggregation
Complex Modeling



And here are (a few) examples of what they produce: