This definitely does not look like pondweed of any kind. To me it looks very much like Chara. Cara is an algae and is best treated with Chelated copper like that in cutrine granular. If you use a liquid cutrine it can work as well but does not get as much contact time. Cutrine is very safe and will not kill your fish. You can always get a fish kill if you kill to many weeds at one time however this is easily prevented by treating one part of your pond at a time.
