Hi and welcome to the forum
Do not replace the filter media/ materials (pads, cartridges, sponges, ceramic beads, noodles, etc) unless they are falling apart.
Companies tell customers to replace the filter pads every week, fortnight, month so you keep buying their products. If you replace the filter media, you get rid of the beneficial filter bacteria that keep the water free of ammonia and nitrite. If you have ammonia or nitrite in the water, your fish can die.
Filter media should be washed/ squeezed out in a bucket of aquarium water. The media is re-used and the bucket of dirty water is poured on the garden or lawn outside.
If you have filter pads/ cartridges, you can buy sponges for other brands of filter, and use a pair of scissors to cut the sponge to fit. You can normally add sponge to the filter while the cartridge is in the filter. If you have 2 cartridges in a filter, you can swap one for a sponge and wait 2 months before swapping the other cartridge for a sponge. Sponges will last 10+ years and only get replaced when they start to crumble and fall apart. I prefer AquaClear sponges but there are heaps of different brands and they are all fine to use. Just find one that fits in the filter or is slightly bigger and can be cut down to fit, and use that.
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The milky cloudy colour of the water is caused by fish food and waste breaking down and no beneficial filter bacteria to clean the water. The best thing to do is stop replacing filter media, reduce feeding for a few weeks, and do big (50-75%) water changes and gravel clean the substrate every day for a few weeks. Hopefully by then the filter will have some good bacteria and the water should stay cleaner.
Your Betta is probably stressed from the water quality, which is being caused by the new filter pads not having good bacteria to keep the water clean.
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Algae is a simple plant that grows anywhere there is light and water. All aquariums get it and you can either wipe it off with a clean sponge, or have lots of live plants to use the light and nutrients, or reduce the lighting times. If you have live aquatic plants in the aquarium, they need at least 8 hours of light per day. If you don't have live plants in the aquarium, you only need the light on for a few hours in the evening when you are at home.
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You should contact your water supply company by phone or website and find out what the GH (general hardness), KH (carbonate hardness) and pH of your water supply, and whether they use chlorine or chloramine in the water.
Most water companies have a website with information (Water Analysis Report) about what is in the water, and it usually includes pH, GH, KG and chlorine/ chloramine content, as well as other things like heavy metals, chemical residue, etc. If you can get that, post it here and we can go through it. Your tap water might be fine to use for the fish and using a dechlorinator with tap water is usually a lot cheaper than buying water from a shop (as mentioned by Levi R).
