Bulkhead connector, yes.
Any connector that is mounted in a panel for electrical pass-through is a bulkhead connector.
These connectors are typically packed with dielectric grease to weatherproof them.
For some reason, it's always brown in color - looks bad, but not indicative of electrical badness AFAIK.
I don't think the color is an issue - just reflective of the product's original color and age.
I'm not sure there would be a benefit to cleaning it.
However, if you want to clean it, I would strongly suggest not to use anything like brake cleaner that might attack the plastic.
I'd use a commercial contact cleaner that says 'safe for plastics' on the label, and an old toothbrush.
I buy the WD-40 brand; seems to be the cheapest:
Works fine.
If I removed all the grease, I would replace it with a simlar quantity of new dielectric grease:
"Dielectric" just means it's electrically non-conductive.
If you want to clean corrosion, I suggest Deoxit brand contact cleaner:
"Technician in a can."
The D5 is 95% solvent and 5% active corrosion remover.
Meant for easy application.
The D100 is 100% corrosion dissolving chemical, and expensive, but you only need a tiny amount if you can apply it directly to the connector.
Use the cheap stuff for cleaning and Deoxit for corrosion cutting, sparingly.
