Blanketweed is a common problem in ponds, especially new ponds. There are various ways to get rid of it, but you should choose your plan of attack carefully.
Your UV clarifier won't kill blanket weed because a UV can only "zap" algae that floats past the light and blacketweed usually attaches itself to the pond walls - it is not an algae that floats about like green water.
Blanketweed thrives of nitrates and phosphates which can be caused by over-feeding your fish. The uneaten fish food can give off phosphates and the fish waste can cause ammonia and nitrate.
Another factor for high nitrate and phosphate levels is "new pond syndrome" - before your pond has had chance to mature with beneficial bacteria, the tap water may be high in nitrates and phosphates. For this reason you should let your pond mature before introducing fish.
Some blanketweed treatments will kill off the algae but then leave you with dead blanket weed which then decomposes and gives off nitrate and phosphate.... guess what happens next....that's right, an abundance of blanket weed!
If you don't have a powerful enough filter for the size of your pond this can be another factor. So check the flow rate of the filter you have.
One of the best treatments for this kind of algae is Swell's Control Blanket Weed, this easy-to-use treatment is comprised of beneficial bacteria to lower the nitrate and phosphate levels and also contains barley straw. When barley straw rots, the microbial process disrupts the algae.