Cutrine Plus granules work pretty well on algae for me, but it is an ongoing year to year issue with many pond owners.
Many like me never really "solve it". And many of the chemicals are not exactly cheap.
I use Cutrine Plus for algae and Navigate for vegetation. Navigate works pretty well too, but again not cheap. I just wear gloves and a mask and throw the granules of both products out by hand. The Cutrine lands on top of the algae and appears to burn it. In a week or so it is greatly diminished, but it may need re-treatment and will certainly need it again next year. Not sure if your state allows Tilapia, but they can be one piece of the puzzle too.
BEFORE CUTRINE PLUS: 
AFTER CUTRINE PLUS: 
