Based on your photos the growths on the leaves appear to be leaf galls created by a tiny insect called hackberry gall star psyllid (Pachypsylla celtidisasterisca). 

These insects are harmless to people and pets and rarely hurt an already healthy tree. This insect along with any other leaf or stem galls are mainly a form of aesthetic damage and do not require management unless someone finds them unsightly. 

The galls are formed by the plant itself as a chemical response to immature psyllid insects (nymphs) sucking up plant juice with their straw-like mouthparts from leaves. The gall growth is made out of leaf tissue and forms around the feeding insect, acting as a protective pocket as the insect feeds and grows through its life stages throughout the summer. In late summer to early fall, the psyllids emerge from the galls as winged adults. Adults will spend the winter months in protected places (under bark, within window screens, etc.) and then come spring, females emerge to lay eggs on hackberry leaves. Once eggs hatch, the immature nymphs will feed, leaf galls will be formed and the cycle continues.

A very unique leaf gall shape (star-like). Something worth pointing out to neighbors and peers so they are aware these galls and other leaf galls are not significantly harming the plant. 

Please let us know if you have any further questions.