If it's a cartridge filter, remove the cartridge and chemically clean it after a good hose-down, thoroughly rinse it to remove any foaming from the treatment, and then put the filter back into operation.
If it's a sand or D.E. filter, start with backwashing and then confirm that the pressure has adjusted back to normal operation parameters. If the cloudy problem persists, D.E. filters may need to be disassembled and cleaned; while sand filters may need a chemical cleansing of the sand media, or even replacement.
If you're not comfortable with performing such maintenance or making related repairs, consult with your local and trusted pool professional to schedule a service call.