Do in decline tokens occupy a region?
E.g.: Forest Clump: Each Forest occupied is worth 1 additional victory coin this turn.
What is the difference between Forest Clump and Heigh-Ho! (Each region with cavern is worth 1 additional victory coin this turn)
Occupied means there is a race token in it. The token could be active, in decline, or a lost tribe. So it just means if you occupy it with either active or in decline races you get 1 additional point.
Heigh-ho just means the cavern symbol is also worth one additional victory point. The difference (going off your descriptions since I don't have the cards in front of me) is that one gives you points for things printed on the board, the other for pieces on the board.