Out of the Green: Before being given a move or close combat action, you may place Swamp Thing in any square of printed hindering terrain within 4 squares.

Hallucinatory Toxins: Swamp Thing can use Poison. When he does, a character that takes damage and has zero action tokens is given an action token.

Nature's Assault: Give Swamp Thing a free action and choose any number of characters occupying a square of printed hindering terrain within 4 squares. Deal 1 damage to the chosen characters.

Two of these effects mention printed hindering terrain. Can I use these effects with hindering terrain markers? Those are printed with a green border, right?
No. Printed hindering terrain is referring specifically to hindering terrain printed on the map. Any other type of hindering terrain (objects, tokens from Smoke Cloud, debris, etc) are not printed hindering terrain.

Does Swamp Thing need to be able to draw line of fire to the square of hindering terrain in Out of the Green?
No LOF is required. If the printed hindering terrain is within 4 squares, it can be chosen.

So does that mean he can move through walls and across elevation?
Yes and no. Yes, Swamp Thing can use Out of the Green regardless of other terrain and elevation. No, he does not move; he is placed. This disinction is important for a variety of reasons, not the least of which is that he can't pick up an object with his Super Strength when placed... you can only pick up objects when you move.