Cutrine Plus (liquid or granular) is only useful as an algaecide, and has no activity toward vascular plants (with few exceptions).
Herbicides (versus algaecides) are needed for plant management objectives.
Endothall (AQUATHOL K liquid / AQUATHOL SUPER K granular)
Fluridone (SONAR, AVAST, FLURIDONE)
Diquat (REWARD, TRIBUNE, et al)
Copper (HARPOON, NAUTIQUE, KOMEEN, et al)
All of the above herbicides and the associated brand names are potential candidates for treating slender pondweed, and many other vascular plant species.
Selecting the proper product depends on the variables associated with your specific pond, as well as your ultimate objectives.
If desired, shoot me a PM and I'll send you a questionnaire to fill out that will provide me with basic but necessary info that I'd need in order to point you in the right direction, in regards to product-selection.