I am not familiar with that HOB
specifically, but from what you said, the dense sponge is the bio media (bacterial colony). You should try to schedule your filter cartridge on alternating schedules. Bacteria will colonize that area as well, and removing both at the same time will greatly reduce your bacterial levels. Also when the sponge becomes very dirty, its ok to rinse it off using tank water. Dont run it under the tap because the chlorine will kill the bacteria. Also please be sure to dechlorinate your water when you do water changes.
You have a very small tank with a light bio load and while being a little casual with your maintenance schedule would probably be OK, it makes sense to get into good habits now because when you grow into a larger tank or you get more fish those things will matter.