Simple script for creating 2d parallax layer in Unity. This is modified from the script in this article
Just attach the ParallaxLayer script to the specific layer that you want to move relatively to the main camera. Then you can set speed in X and Y direction from 0 to 1. The farther the layer is, the higher the speed. So a layer at infinity will have speed of 1, while the closest layer will have speed of 0.
There's a TestScene for an example of how this work.