Like I always say, make sure its chara! Chara will have a gritty feel when rolled in your fingers and emits a very strong musky/skunky smell when pulled out of the water.
I typically use Algae Defense (chelated copper) with very good results. Spray it on at about 1 gal/acre foot and you should be good.
The harder your water, the more chemical you need to use. I'm assuming theres no trout, but trout and koi can be REAL touchy with copper products, but usually if your hardness is over 50ppm youre in the clear
Aeration is also a huge factor when treating ponds. It can be your lifeline when you need to run high rates of pesticides in the heat of the summer when oxygen levels are low.
Ive also had real good luck with granular copper sulfate at 2.5 lbs / acre foot. Get the large grains if you can--less dusty and easier to broadcast.
