small green insect with "helmet"

That is commonly known as a Buffalo treehopper. There are presently at least 16 species which belong to its genus Stictocephala.

These are sap feeding insects and generally do little or no harm to their given host tree or plant.

Only a few species belonging to this genus are "stand out" species; otherwise the rest of them can be "taxing" to tell apart unless you have them looked at by a specialist.