It looks like a cloud filter which the person used the square selection tool to random, adjust the color and/or saturation of at various sizes and locations. Maybe another cloud on top set to screen to give the color variation.
If you are looking for a quick way to make a similar effect create a diference cloud. Use the mosaic and/or patchwork filters. tint to desired look with hue/saturation and put another cloud on a layer above set to screen for the color variation. I was able to create a similar look in 30 seconds.
Linear Mode
