I worked my way through college running a pool company. As others mentioned, you have DE there on the bottom. This again comes from a tear in the crescent shaped filers or one of the filters is not seated correctly in the manifold. Super simple to take apart inspect and replace bad filters or reinstall if there's a seating issue. Make sure you reinstall the filter lid carefully and completely. Those are under pressure and I've seen them blow 50 feet in the air when the band clamp fails. Typically about a quarter to a half inch separation on the band clamp when tightened will do (don't over tighten). Step back after turning the system on and observe the pump/filter getting primed and pressurized.
The skimmer should be at least 50/50, sometimes more when the wind blows to keep the pool clean and water as healthy as it can be. Chlorine sanitizes, filtration keeps the water nice and clear. Skimming breaks the water tension where bacteria builds up and allows the chlorine to do its job. Keep the chemicals balanced via testing, never too much acid (!) and add conditioner once a year which allows the chlorine to do its job and not dissipate right away. Done right, pool water is more bacteria-free than tap water (assuming all the kids in the neighborhood aren't swimming in it, lol).
Oh, and they are a PITA.
Happy to help via PM if you have any more questions.