If you have algal growth present, Alum is not going to take that away. It's tied-up nutrients. Suspended sediment is one thing, algae is another. If you want to lock the algae up you have to kill it so nutrients can be released back into water so you can lock it up with Alum. Flock on the surface is normal in a pond situation that blows to one shore and slowly disappears. The test in a jar is not going to be the same thing you see in the pond.
