Dielectric grease keeps out water etc, but it is also an electrical insulator.
The problem with plastering it all over the outside of a connector is that when you unplug the connector, some of it gets on to pins and into the sockets, and then causes bad connections.
The best plan is to remove as much of it as possible before you unplug the connection. Cleaning the electrical contacts may be difficult, because the main purpose of the grease is NOT to dissolve in or mix with any liquid it is likely to come in contact with!
There are "industrial grade" cleaning fluids that work, but you won't be able to get those easily and using them incorrectly is a serious health hazard.
The best option is probably "rubbing alcohol" (US name) or "methylated spirits" (UK name), but note that while a product sold as "meths" should contain at least 90% alcohol by volume, "rubbing alcohol" may be as low as 70%, so search for a brand with the highest alcohol content you can find.
"Absolute alcohol" which is at least 99% pure is an even better option, if you can get it. Try a pharmacist, if you can't find it for sale anywhere else.
Note: these products may contain either ethanol or isopropanol (isopropyl alcohol). For use as a cleaning fluid, the difference between the two doesn't matter.