Could be many different things (including nothing serious, like leaf burn) but keep an eye on its progression.
As seen on Google:
"Chinese elm is susceptible to anthracnose, which results in irregularly shaped black spots on its leaves. If the disease is allowed to progress, it eventually causes leaves to turn brown and drop from the tree. In a most serious case, the tree develops patches on its branches and trunk. An afflicted Chinese elm in a cool region may take several years to die while a tree in a dry region won't die from the anthracnose. The best management practice for the problem is the removal of infected tissue, cutting each affected branch below its wounded area and removing the tree's litter from the ground. Replace the tree if its condition becomes too bad."
