Reaction-Diffusion can be seen anywhere in nature, from patterns on fish, coral, to zebras, and anything spotted. Doing this in photoshop is extremely easy and an old trick :) You just have to have an initial image, then apply a set of filters that "cause a reaction" (sharpen, highpass, levels, edge detect... anything that brings contrast) and then have it diffuse (blur, median, min/max, and otherwise so many options.)
There are so many things you can do with this one simple texture and it takes no time at all to generate them.
I also recommend experimenting with new chains of reaction-diffusion patterns like edgedetect-->threshold-->median and get a bunch of patterns we've never seen before.
This is an old but useful trick for texturing artists. Hope you enjoy the video