1. First use "Cellular" map, animate the "Spread" will make the circle grow
2. Tick "Fractal" so the pattern become more organic or natural.
3. In Output, Enable Color Map, make it more contrast.
4. On top of that Cellular texture, add ColorCorrect (a free plugin from cuneytozdas), this will normaize the output of texture map, so that the range of value is suitable for "Opacity channel" in a material.