I've used barley extract and barley bails before, but I don't think they really did much. I think the best way to get rid of algae is to add lots of plants. Floating plants are great, but other plants should be planted directly in the pond without any pots. If you can, grow them in between rocks as long as the roots can get to the pond water. If you can't do that, the next best thing would be plant them in mesh bags. You can buy them or make them.
The theory on the plants is they compete against the algae. Basically starving out the algae. The algae grows due to excess nutrients in the water.