Whenever I change the filter I notice that there is a bunch of fuszzy white growth that looks suspicously like mold. I checked all the water params, and it's all good. I am jsut worried it could hurt my fish. Has anybody else seen this or know what it might be and/or how harmful it is?

When you say "change the filter" do you mean when you rinse the media out in a bucket of tank water, or are you actually changing the filter??
Is it possible that the filter is sucking in fish food? That might cause mold to grow.
Mary

As above..Do you mean filter sponge? You only need rinse them. If you change it you will damage your cycle. I expect you know that - the stuff you see on the filter is gunk sucked in by the filter which starts to decay & should be rinsed off in a bucket of tank water or dechlorinated water and then the sponge can be placed back. I think its just doing its job If its white maybe you have chalky water- high limestone content.

I have an Aqua Tech 5/15 filtration system. I have to change the blue filter cartridge ever couple of weeks, but there is a BIO-fiber that I don't change. That is what is grwoing al the nasty white stuff on it. I was told to never clean or replace it by hte manufacturer, so I don't, but now I am worried. Is it okay to give it a quick rinse in dechlorinated water?

I would rinse in the old tank water outside the tank.

Yes you have to rinse them too just a few swishes will get rid of the large particles. The good bacteria will not be lost.

The blue filter sponge- is that a wool mat? You can rinse and reuse those too. It doesn't have to be a thorough cleaning just a rinse in the old tank water or well-de-chlorinated water Every 2 or 3 weeks I do mine