Hmmm, you might want to check the bulb
of your UV light. I don't have one but I know
UV ponders suggest this. The bulbs need
to be changed once in a while.

Other than that, here is my standard
green water answer ;-)

Green Water is caused by single cell algae.
Algae thrives on sun, fresh water, fish waste, fertilized run off,
rotting plants and blown in dirt. In new ponds and spring ponds algae
is always the first thing to start growing.
The best defense against algae is to have lots of plants to compete for
the nutrients, few fish, not overfeeding those fish, some shade and
cleaning up debris.
So not use algaecides, they only make lots of suddenly dead algae to
feed the next algae bloom. Gently remove string algae. Don't worry
about fuzzy algae that grows on the sides of things.
You can purchase an Ultra Violet light set up to eliminate suspended
algae, running the water under the light. It does not work on string
algae.
Filtering with plants works well. I run my water into a stock tank
topped with water hyacinth and down a waterfall planted with
watercress.
Most algae blooms will pass within a couple of weeks.
Time and patience is key.
Remember patience...

kathy :-)