The reason for this happening is that the cactus is at least 2 blocks tall, so the hearthling can’t simply jump down – they can only go up or down one block at a time unless there’s a ladder.
I think they will be able to walk across the tops of the other cactus, so you can simply build a ladder beside them in order to let the hearthling down. However, that in itself is part of your bigger problem (and how the hearthling got up there in the first place): even though it looks like there’s space between these cacti, their hitboxes are actually solid so there’s no space for hearthlings to walk between them. When the hearthling in the picture was placing their cactus, other hearthlings must have placed catci behind them and blocked them in. Because the hearthling was “squeezed” out by the solid block of the cactus (remember the hitbox takes up all the cubes that the cactus is in, not just the part that’s visible), he couldn’t escape through the sides… so up he went!
If you want your hearthlings to be able to harvest all of the cacti, you’ll need to move them further apart – make sure to leave at least 2 blocks between them so that the hearthlings can fit through. The good news, though, is that if you move the cactus which the hearthling is stuck on, then it will stop being solid for a moment; he’ll fall to the ground and go eat something!