I agree with all the above. How are you cycling, with fish or fishless?
If fishless, then I wouldn't do anything about the gunk. It's just diatoms, and will go away on their own. If you disturb it and spread it around, it will just get more on the bottom. Let it run it's coarse. You also don't want to do PWC
's in a fishless cycle unless your levels are just too high and need to bring them down. And if you have fish, well, then you need to keep ammonia and nitrite down below 1ppm, preferably .5ppm or lower.
Once the cycle is complete, then do a 75% PWC
to bring your nitrate levels back in check and add fish. Then, during that PWC
, you can remove as much of the gunk as you can. Generally diatoms will last a few weeks to a couple months, then will completely disappear. So it's nothing that will be permanent. It's common in all new tanks.
As for the filter, most of the media should only be rinsed in a bucket of old tank water. If you replace, then you lose the bacteria. Sponges can just be rinsed in old tank water as well, and any other filter pads. The only time I ever replace anything is if it begins to fall apart. I haven't replaced anything yet, other than filter floss, as it takes years before anything gets so bad that it needs to be replaced. And if you use ceramic rings or cylinders, or lava rock, you'll never have to replace them.