PineTime charging issues - connecting pins black

[tllim]

I've been having issues with charging a PineTime lately.

The issue seems to be physical: the connecting charging elements on the PineTime are blackened... I'm not even sure if they are just "dirty" (some sort of accumulated/solidifed filth), oxidated, or what else.

See picture: https://imgur.com/a/1GlklJP

I'm a bit concerned about trying to forcefully clean it off, as I'm afraid I may be damaging it.

Is this a known issue and are there suggested best practices for dealing with it?

Looks like oxidation, use pencil eraser may able to rub off the rusted surface.