I would worry about brake cleaner on plastic. Do a test on the plastic somewhere that any damage won't matter. Instead, I'd use contact cleaner -
https://www.amazon.com/WD-40-Specialist ... ct+cleaner
Throttle body / MAF sensor cleaner would also cut the grease, but again I would test on plastics first.
JMO - the old grease may have discolored, or it may have been that color from the start. I don't think it matters what color it is - if you can overcome the urge to clean it, you'll save a lot of time and mess. If you have a melted connection, or a high-resistance or open circuit at the bulkhead, you might want to clean it. Personally I would simply run a wire through the bulkhead connector ala this - http://madelectrical.com/electricaltech ... ges2.shtml
Dang - I thought I had invented that wire-through-the-connector method, but no.
Note - the connectors in your car don't add much value for the owner. They are there to make assembly of the cars on the line fast. I would not hesitate to remove or bypass a melted or flaky connector with solid wire. If you later need to separate the harness pieces, just cut the wires and them splice them back together when you assemble. Connectors are a maintenance headache,put there only for the convenience of the factory.