Just get some Cutrine Plus Granules...approved for aquatic use.
Don't treat the whole pond at one time because too much at once can cause a fish kill.
On a non-windy day throw out a few handfuls of the granules to land on the top of mat of algae...I use a gloved hand....treat about 35% of your pond. Wait about 10 days to two weeks and then treat the next 35% of your pond....and so on.
Cutrine Plus Granules It's worked well for me...see my before/after pics:
