there are so many factors to this question, how big is your filter? do you do maintainence dosing of bacteria? are you in full sun? do you use barly products? how many fish are in your pond? how quickly is your pond turned over thru the filter?... heres some basic tips that will help...
cut down on the sunlight.. shade the pond, use pond dye, lots of plants(they shade the pond and compete for the same food the algea eats
cut down on feeding your fish or even stop for a week.. this will help cut down on nutrients in the water
if you fertilize your plants.. make sure its not bleeding into water
use barley products... preferrably extract
my 9 watt tetrapond u.v. cleared my water in 3 days, but it wasnt totally green(pea soup) my pond at the time was 500gals
maybe your filter is just not big enough
this may help... i hope so ... GOOD LUCK!!!