The blue sponge is called matala matting, it very course, stiff of plastic meshing, its just a form of filter to make sure on stray bits like algae go into the return chamber and block the return pump, it also helps house a huge abundance of pods, sponges, squirts ect, pod migration is pointless, the return pump will mash anything taken back up to the main tank. Luckily i have 105kg of live rock, so that keeps the population going in the main tank. Its also lit 24 hours by a D&D twin T5 lighting unit, abit more than it actually needs, as navarre said, they can be lit by a regular house bulb with no differnce at all!.
leon