Algae like this is usually a symptom of a water chemistry issue - likely too much nutrients in the pond and/or the pond is getting too much light. Things you can do:

  • Test the water for nitrates and phosphates - if you have much phosphate content then you need to figure out where it's from. At this time of year fallen leaves provide a lot of nutrient input to ponds, often causing algal blooms, particularly in small ponds and those which don't have a nice water chemistry balance.
  • If you're getting a lot of leaves into the pond, consider putting netting over the pond to catch them. You will of course have to regularly take the netting off and shake it free of the leaves somewhere that the leaves won't find their way back to the pond.
  • If the pond hasn't got much surface cover (shade), consider planting a tree (or more than one if it's a large pond) but keep in mind that trees take time to grow and they drop leaves in the autumn.
  • Alternatively, floating plants like water lilies will help provide cover in the growing part of the year but they do disappear and rot in the autumn and winter, so dumping nutrients back into the system themselves.
  • You can actually buy bags of hay (yes, really) that you place in the pond and the hay releases a substance that helps reduce algae and/or keep it at bay. This is a very eco-friendly way to approach an algae problem and is often quite effective. You do need to switch the bag out every now and then.
  • If all else fails you could consider chemicals but you should get to this as a last resort - anti-algae chemicals can have detrimental effects on other things and they are almost always a short term remedy - you need to fix the underlying problem.

That's all I can think of right now.