Reaction Diffusion
Two chemicals spread across a grid and one of them eats the other. Draw on it to start something off, then move around the map: coral, fingerprints, spots and worms all fall out of the same two numbers.
Most of this map is dead: whatever you draw there fades out within seconds. The living part is the pale band along the curve, and every named spot sits on it or just beside it. Tap anywhere to go there, or drag to travel without letting go.
Brush, walls and detail
Nothing crosses a wall and nothing grows inside one, so you can fence off a shape and let the reaction fill it. Walls cost a little speed and go away again when you clear them.
A finer grid makes smaller features, because the pattern has a size of its own in grid squares and not in pixels. Automatic starts low and climbs as far as the frame rate allows.