This is happening to only one of my trees, it's not brittle but very soft to the touch, almost velvety spongy, any ideas?

I've got some that look like that due to spider mites and drought. There are likely other causes. If it's just the older leaves that's not serious.

Hi,
Lack of water ... IMO -especially due the wilted and sad look of the leaf.
Try to give it more water. How often have you been watering for now ?

Everyday, it's in a self watering pot

I would say potential root rot.  The pot may be staying too wet.

Hi,
I would check if the dirt wicks properly ...
If you have water in the reservoir but the tree can't access it because the potting mix is not water friendly ...