It is non-conductive, but when you slide the connectors together there is metal-to-metal contact and the grease gets displaced but the area around connector is protected from moisture.
Using a bit more grease is usually a wise idea, since it protects the connectors from corrosion
by displacing water
and reducing exposure to oxygen. If the contact between the two pieces is such that enough grease is between them to cause a problem—you've got other problems with the connection (that's when conductive grease saves the day). The connection should be tight enough that it forces the grease out of the way.