Blanket weed can be a real problem at this time of year when the water warms up during warm spring days. The rapid algal growth occurs because the water is rich in nitrates, which is caused by decaying vegetation, over feeding of fish, or fish waste. Unfortunately small ponds are particularly vulnerable.
Often the problem sorts itself out as the pond plants start into growth and begin to use up the nutrients. However if it persists then I would recommend that you try using Pond Wizard.
There are a few other measures that will help. Create shade around the pond by adding more plants. Try adding a small bundle of barley straw, about 50g per square metre. As it deteriorates it releases hydrogen peroxide which inhibits the growth of the algae. You can always remove the weed by hand - just wind it around the end of a garden cane and you will be amazed at how much you pull out. Remember to leave the removed weed beside the pond for a day or too before you add it to your compost heap. This will allow any pond life that is trapped in the weed to make its escape back to the safety of the pond.