I specifically use Propeller aquatic herbicide for water meal and other unwanted plants in and around my pond. I use copper sulfate to treat the algae. A good aerator is greatly beneficial when doing this as the dying vegetation with deplete the pond of needed oxygen.

Propeller seems to be discontinued from ThePondGuy.com but here is another link to the same product.
https://www.forestrysuppliers.com/pr...

Copper Sulfate can be purchased at any Tractor Supply.
https://www.tractorsupply.com/tsc/pro...