Cloudy water is typically due to insufficient chemical treatment or an inability for your filter to remove all contaminants from your water.
Chemistry:
Check pH, calcium. Incorrect levels may cause cloudiness.
Chlorine – if no chlorine is present, shock pool. Cloudy water can occur when chlorine demand is outpacing chlorine supply – there is not enough shock to kill and breakdown all the debris in the water.
Stabilizer – extremely high levels of stabilizer can cause chlorine to become less effective, driving up demand beyond typical 3-5 ppm levels.
Filtration:
Sand – Backwash filter, add Fiber Clear, run for 24 hours minimum
Cartridge – Clean filter, add Fiber Clear or Ultra Clear, run 24 hours minimum.
DE – Manually clean DE elements, recharge with DE  add Ultra Clear, run 24 hours minimum
As your filter removes debris from the water, it will become dirty again.  If you filter plugs up quickly, and shows signs of captured debris when it is cleaned, what you are doing is working.  Continue until pool is clean, filter no longer seems to pick up debris, or pressure gauge stabilizes around 12-15 GPM.