I'm sorry I haven't responded to that question about the gooey stuff, too. I'm sure what you are talking about is just the filmy white-ish algae that forms on the filter outlet, also in the foam or mesh of the cartridges, and in the intake tubes.
When I clean my filter I wipe that stuff off and use a flexible brush (made for cleaning tubes and hard to reach areas) on the intake tubing, and down where the impeller goes, and on the impeller blades, because eventually that gunk builds up.
My Aqua Clear 70 HOB filter has a three stage filtration system. A foam block, a carbon bag, and a bio-ball bag.
When I clean my filter, I gently rinse the bio balls in old tank water that I've siphoned out, and I really squeeze and work out the goo on the foam block, also in the old tank water.
The foam block can get really clogged, but I only want to get the gunk out but not kill off any bacteria that is in it. The bioballs I have never replaced, yet... I just keep rinsing them gently in old tank water.
In an Aqua Clear you are supposed to replace one of the three filtering components on rotation. It comes with a foam block on the bottom, a carbon bag in the middle and a bio ball bag on the top.
I don't use the carbon all the time, but if I do it loses it's action after a couple of weeks anyway and becomes a good surface media for the bacteria, too.
If I need to replace the foam block because it is too gunked up to get fairly un-gunked, then I do replace it, but I rinse the new one out in tap water, then put it in a bucket with some tank water and Prime, and make sure I've removed any chlorine that might be in the block.
I have room in the top of the basket to stuff some extra polyester foam, or another bag of bio-balls, just in case I need to seed another tank with healthy bacteria.
I hope this helps on the goo and filter cleaning question.
You've come to the right forum to ask about African tanks. These guys are pros and have awesome tanks.