This is not white spot. The fish is covered in excess mucous, which is caused by poor water quality or external protozoan infections.
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Your water is way too soft for them.
Mollies need a pH above 7.0 and a GH above 250ppm.
Swordtails need a pH above 7.0 and a GH above 200ppm.
If you add some Rift Lake water conditioner (mineral salts) to the tank and raise the GH to about 150ppm, it would help the livebearers (swords and mollies) and wouldn't be too high for the other fish. However, the best option would be a second tank and put the livebearers in one and raise the GH to 250ppm.
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Until then, add 2 heaped tablespoons of rock salt (aquarium salt) for every 20 litres (5 gallons) of tank water.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
