I think your problem is with your corrosion control system, not the way you have the boat zinced. Over-zincing can be a problem on a wood boat. But so far as I know, it's not a problem with a fiberglass boat, but I could be wrong about that.
On our boat we have two transom zincs ("license plate" zincs), two shaft zincs on each shaft, and shaft wipers.
Because our harbor is in a bay with a fair size stream entering next to the harbor and a large-ish river entering a couple of miles away, the upper surface of the water in the harbor is often fresh or brackish. So we, like a lot of people in our marina, hang a zinc on a cable off the side and down about six or eight feet. The other end of the cable is connected to the boat's bonding system. This ensures that there is good connectivity between the bonding system and the surrounding water even when the upper layer--- the layer the transom zincs are in--- is fresh or nearly fresh water.
We don't have any sort of electronic corrosion control system other than a galvanic isolator if that can be considered "electronic." So we don't have any system that gives us warnings. But our boat is set up the way almost all the similar boas are in our harbor and the setup and our hanging zinc were recommended by the marine electric shop we use. We've not had any problems in the 17 years we've owned this particular boat.
So in your case I would ignore the warning from whatever your corrosion control system is. The whole subject of corrosion and electrolysis control on a boat has an element of black magic to it anyway. So I would be inclined to not put much stock in a "control" system since I have no idea if they are programmed to account for the twenty million variables in corrosion control.