Green is not a big problem...but get some good light on the area and inspect closely for any sign of pink which is a big problem.
If the zinc has not been changed since last spring...you are overdue for an inspection but with cold water and NO pink...you can probably hold off a bit. If there IS electrolysis going on, you can hang a zinc fish on a wire from a grounded part of the boat to overboard to try minimize damage but that is no guarantee.
The only way to truly protect the shaft is with a shaft zinc...which means a diver if you are still concerned after a close inspection.
No longer posting. Reach me by PM!