Usually any repair to a gauge is a one way trip, to the trash. Any attempt to repair usually just breaks the gauge. With condensation, you may be able to uninstall the gauge, and leave it in a sealed bag of desiccant (white rice works well) for a couple days/week.
If the tach works normally when it is running, I'd just ignore it when the key is off! That isn't anything you can fix without replacing the gauge internals.