Have this sick Dendrobium...

[King_of_orchid_growing:)]

Is that moss for the potting medium?

I'd avoid using moss for growing Dendrobiums. They usually can grow fairly dry. I grow many of my Dendrobiums mounted on cork bark without any moss underneath the roots outdoors.

I recommend using small or medium grade bark as the potting medium.

You might want to check the roots. It might be good to show us a pic of the roots too. You might be able to get some advice as to what is going on with them and what to do.

Right now, based on what I'm seeing in the photo, the pot's too big too. I recommend a much smaller pot size (probably 1" or 1.5" smaller, if possible).

If the plant is top heavy, put some rocks that don't absorb a lot of water on the bottom, and clip the plant in place with some wires (be creative with the wires - it might take some trial and error).