what type of filter do you have?
I have 2 aqueon quiet flows and I asked about filter cartridges on here before. I was told that you do not need to change them monthly unless they are so plugged up that the water is no longer flowing through the cartridge. Most of the bacteria that you want lives in the filter. It was suggested that instead of replacing the cartridge monthly, swish it around in old tank water when you do water changes.
I "think" most of what you are seeing is the beneficial bacteria growing on the cartridge.
For a fully cycled tank that has completed the nitrogen cycle - you want to do about 50% water changes a week to maintain water quality with light substrate vaccuming. You dont want to vaccuum too much all at once because the same beneficial bacteria also live in the substrate, on the walls and on any decor in the tank. However the most of it is in the filter.
Dont rinse it in tap water, that will kill off thebacteria.
If you happen to have a marineland pengin bio wheel filter, then the bacteria will be on the bio wheel and I have been told that you should never replace it, unless it is broken. I'm not sure about their catridges...I think its more to collect debris.