Green water (water born algae) usually occurs during bright and sunny weather, but it is not necessarily a bad thing. Ok so you cannot see your fish, but fish are often happier in these conditions for that very reason, knowing that they are safer from predation. Most natural ponds turn green and hazy at some point during the summer months.
The attraction of having a garden pond is that you bring a bit of nature into your garden and yes, you would like to be able to observe it.

How do I remove it?

A quick solution is to add a green water treatment such as NT Labs Magiclear to your pond. These flocculent treatments will clump the green water cells together, clearing the water and they sink to the bottom, or the pump will draw it away and pass them into the filter. These treatments often need to be repeated on a monthly basis.

After using a green water cure, we then recommend using a sludge reducing treatment such as Waterlife Bacterlife which will biologically break down the dead algae and digest organic material. This will lead to a cleaner, healthier, pond and filter, leaving fewer places for disease-causing organisms to hide.

Is there a more permanent solution?

Yes there is. For a more permanent solution, you can fit an ultra-violet clarifier (UVC) to your pond filter system. These basically burn off the algae cells with the UV light and the remains are collected and broken down in the biological filter. Lamps inside these units need to be replaced every year.