dust

Your MacBook Pro  uses ‘smart charging’ to charge in the optimum way, and only when necessary. Plugged in is Not necessarily actively charging. There is substantial hardware and software cooperating on battery and charging issues. Simply asserting a charging voltage against one of the interface(s) will NOT successfully charge your MacBook Pro.

In general, Best Practice is to ALWAYS connect AC power when it is possible to do so, and only run on batteries (which could be somewhat slower) when no AC sources are at hand. Your Mac will NEVER over-charge. 

Charging the battery does not produce dust (or anything else for that matter).

Environmental dust is everywhere, and accumulates regularly.

The unreachable area of the floor under the ball-and-claw tub, or behind the toilet tank, if so equipped, would explain this to you right away.

The only issue you are seeing here is that it is accumulating on shiny items you examine intently.

Unless the seal is airtight, dust can get in anywhere, and usually does.