Darkest Dungeon – What Does the Green Outline of ‘Preferred Position’ Markers Mean?

At this point I remember six icons.

1. Wooden Door. This is where you entered the dungeon.
2. Faded (low-intensity) cavern-looking thing with brick walls. This means empty. In the OP's screenshot this was more obviously a brick dungeon room; in the current build the walls seem blank but the bricks are still visible at the bottom.
3. Faded goblet. Til I figured out it was a goblet, this to me looked like a monster holding its weird hands over its head; like a freaky scorpion or something. Nope. Goblet. This means treasure; usually from a fountain or some other interactable object. I'd guess that goblet loot skews to provisions and buffs.
4. Golden chest. This room also means treasure, and (almost?) always comes with combat. Treasure is in a chest, which may or may not be locked. I'd guest that chest loot skews to heirlooms, trinkets, and gold.
5. Three red figures, slightly slanted. There'll be a room battle here.
6. A red skull. This is a boss room.

Note that once a room has been cleared, it will change to the #2 Empty icon.

There might be other icons; as I play I'll refine this list.

Both the Backtracker trait and the additional stress penalty refer to walking backwards within a hallway after having walked forward. I couldn't find an official source for this, but it's relatively easy to verify manually that walking backwards through an entire area causes more stress than exiting, reentering, and walking forwards in the opposite direction. (If you want to verify this, I recommend using throwaway characters)