There are 300 cell types in each genome. Each cell type has three values for three possible growth directions (forward, left and right). If -2 is written there, then it does not grow. If there is -1, a spore grows there. If a number from 0 to 299 is written there, then it grows and the new grown cell will be of the same type as this number. That's all. This defines the entire pattern.
When a mold dies, some of its spores turn into new little molds. A single number in their genome can be replaced by a random one.