Green pond water is a common complaint, especially during the summer months. The reason pond water goes green is because of a growth of algae. Pond algae feeds on the nitrates produced when bacteria break down fish waste, and sunny conditions can make algae grow very quickly indeed.
The main ways to prevent the spread of algae is to remove the fish waste, by scooping out the sludgey layer at the bottom of the pond, and by using algaecides which are chemicals to kill off the algae. These chemicals are dangerous to use when you have pets and children however, so do be careful! You will need to read the instructions carefully, and check if your pond fish will need to be moved into a temporary home while their pond is being treated. Some chemicals could harm your fish.