There are a lot of different factors which contribute to a green water issue and it may not be the UV system at fault.
The steriliser will work to maintain rather than transform water quality. You may need to add treatments in order to clear up poor quality water or, consider a heavy water change. Algae growth in water can be due to high phosphate and nitrate and green water is heightened in the summer period due to the extra sunlight, so it is also advised that a water test is carried out.
If the UV light is working then the product is functioning correctly.
If you do need to replace your bulb, please check what size your unit requires and you can find our range of bulbs listed here.
Remember, algae and blanket weed are two different factors. Please identify which of these you are experiencing before trying to treat.