I don't know Zinc Oxide, but sometimes the algae on the side of my ponds get very long. I always thought of it as a guide for feeding. My thinking always was that when it gets to long you are feeding more than what your filter can handle at that stage. What I normally do is not to feed for a day or two. They get hungry and eat the algae. Then I feed a little less until my filter can keep up with my feeding habit. The other thing in my opinion is to check your plumbing for places where it might suck air, if this algae is more and longer close to the water return. I might be wrong but this is my opinion.
"Legacy of Koi Pride."