Gray/Cope's Gray Tree Frog - Hyla sp.

Cope's gray tree frog (Hyla chrysoscelis) is a species of tree frog found in the United States. It is almost indistinguishable from the gray tree frog, Hyla versicolor, and shares much of its geographic range. Both species are variable in color, mottled gray to gray-green, resembling the bark of trees. These are tree frogs of woodland habitats, though they will sometimes travel into more open areas to reach a breeding pond. The only readily noticeable difference between the two species is the call — Cope's has a faster-paced and slightly higher-pitched call than H. versicolor. In addition, H. chrysoscelis is reported to be slightly smaller, more arboreal, and more tolerant of dry conditions than H. versicolor. Microscopic inspection of the chromosomes of H. chrysoscelis and H. versicolor reveals differences in chromosome number.

I think this is Hyla chrysoscelis, but can't be sure. It was tiny (maybe 15 mm long) and green on its dorsal surface. I spotted it on milkweed that bordered a pond.