1 part white vinegar and 2 parts water in the moka. Let it sit for 40 minutes. Heat it for a cycle. Heat and steam will clean it. Some believe a layer of coffee oil enhances flavor.

I tried this and still black… vinagre and water… also w sodium bicarbonate. No coffee oil though….

Bar Keeper's Friend, the powdered kind. Sprinkle and clean with damp paper towel, then wash with regular dish soap.

This is stainless. You can sand and polish it. Wet/dry 2000 grit sandpaper used with water can sand it. Bonami or Comet or toothpaste are mild abrasives.

The issue is that it’s inside the chamber so it’s not easy to maneuver in there

Boiling hot water and a dishwasher tablet. Sit for 2-3 hours or overnight. We use this method for coffee stained thermoses.

Any stainless steel or aluminum has to be cleaned. You should have no problem getting your hand inside with the BarKeeper and a wet paper towel. I clean mine almost every time I use them.

ETA: Yours looks inside, like my egg poacher. The water turns the pan. It will not affect the taste, just the look. The bottom is a reaction to grease.