Separate species are known to evolve to be similar if they live in similar conditions. We've already covered the evolution of intelligent plants here, and if those intelligences live in similar conditions to humans, it's possible for them to evolve similar characteristics. A stretch, but technically possible. The dryads and humans are actually very closely related, and only recently have the groups split and evolved differently. Maybe humans lost their plantlike characteristics, or dryads evolved photosynthesis on their own. It can be evolutionarily advantageous to look like another species. Perhaps the more "people-like" plant-organisms scare off predators, and survive to reproduce through generations. Green hair and color change may be artifacts - technically, atavisms - from earlier forms. Lichens are organisms consisting of both fungi and algae (or other plants). The fungi provide a structure while the plants produce energy, and together, they act as one organism. Perhaps one offshoot of humans developed a similar relationship. Many organisms on Earth contain genes that they did not evolve - genes that were placed by viruses or bacteria. Although unlikely, it's possible that your dryads and humans evolved completely separately, but similar transfers began to blur the lines.