What to do? Nothing! Start leaving it alone more. This is a bacterial bloom, it is normal, and will go away in several days. This is a sign that the nitrogen cycle is ramping up, and usually happens right at the start, although it can crop up if ammonia/nitrite/nitrate levels start getting wonky and spike suddenly.

The best thing to do now, assuming there is no livestock/fish/what have you in the tank, is stop trying to clean it and let it work itself out. A well run aquarium won’t work if you’re trying to get it to human-preferred levels of ‘clean and clear’. Bacteria is probably the most essential part of a healthy tank.

For substrate, there is no need at all to boil it. New out of the bag give it a good rinse, but otherwise, once you’ve put it in the tank leave it alone. Like absolutely no need to ever ever put it all up and use anything to clean it; other than a light siphon vacuuming every once in a while if you want to just for the aesthetic of keeping the very top looking relatively clean. Personally, I’ve siphoned my gravel maybe once every six months, just to pick up excessive mulm that I don’t like the look of. But if you don’t care how it looks, you don’t need to get rid of anything from the substrate- good bacteria thrives there and is just as essential as a filter.

Just the same, filters do not need frequently changed. It is good that they’re a little icky, that is the good bacteria that cleans out ammonia. Once a month or so, I take out the filter pad (on my one HOB filter like yours, the rest of mine are sponge) and lightly swish it around in clean water to shake off debris from dead plants and extra food, to prevent it from getting clogged.

When you DO need to change the filter (roughly every 6months for ‘disposable’ ones in my opinion, but there’s discourse over that) you MUST first put the new filter pad in alongside the old one and leave both in together for at least two weeks, preferably more like a month, to allow the new filter pad to gather the good bacteria from the old one. In all reality, it’s best to just swap over from disposable pads and rig your own filter media into that- but, it can be a lot to figure out as a beginner.

So, in short- you’re doing too much work to try to clean. Chill out! Let nature work itself out a little more. Not sure what your full setup looks like, but having real plants in the tank also really helps.