If you leave the foil on the rock, you won't wash away the concrete you just laid when you foil the next section. After you remove the foil, use trowels, brooms, brushes, branches, whatever to put in the cracks, and textures you see in natural rocks. The professionals use molds from real rocks for this. I'm not a big fan of this technique. Don't get me wrong, it works, but there are better ways to do this.
Mussels shoot eggs into the water to spawn.
These eggs will end up in the waterways everytime your pond overflows. Nature doesn't need them. As an example, the zebra mussel. These things made it into the Great Lakes as ballast in a cargo ship. They have no natural predators here.