Crassula species I would say. Possibly C. ovata.

I agree that it looks like Crassula ovata, aka jade plant.

"And is it supposed to grow like that?"

No :-) Succulents like this require a great deal of strong light. Generally any leaning or extended, loose growth is the result of inadequate lighting and the plant leans in the direction of the strongest light.

Thank you for the suggestion, but I feel there's still some difference, the leaves look more elongated and thicker and the stem doesn't look alike. I do have a C. ovata (yellow circle on pic below) and it occupies the same spot with my mystery plant (blue circle). Since I mounted the rack on a sliding door, when the door is closed the plants there would receive the most light, and hot afternoon sun. My C. ovata grows okay, the other one not so much.

Ooh, I think that's a good match, Rina. Thank you.. :D

Agreed. But it is Senecio jacobsenii.

The separation of this to Kleinia is not widely accepted.

Agree. W/o pairs of opposite leaves, it can't be C. ovata.