Get a soaker hose, ring it around the tree under its shade, run it for a couple days.....    

Hackberry's  are tuff tree's, don't think they are in trouble.   To me, tree's are like crops, if you take extra good care of them, they will take extra good care of you.

 FWIW:  my hackberry's are showing the same.... grass is just starting to fade from green.   :(   Not sure if its from water stress, but it never hurts to keep a good tree from thirst...... no matter what size.