The idea was to have it's racial ability be effected by the ingame day/night cycles, climite, weather, atmosphere, etc.
So for example, at nighttime, might suffer a small decrease in regen, but during the day have an increase. During dawn/dusk, it be normal. Maybe cold climate effected them greater than other races, same for heat, but in other environments they aren't effected as bad. Maybe in atmospheres where you need to breathe, they aren't effected by it because they don't breathe. so many different ways you can play with this idea.
I like the idea of making them look human/elvish, but having barks on their feet and hands. Maybe vines running over their body or something.
I've played GW2 and do not see a them as the Sylvaries from GW2. They both plant races.. that about it.
The night/day cycle is interesting, however I wouldn't make it a "detrimental/benefit" system from night to day, but each time of the day would include a slight different setup
Example, taking some of your own : During the day, more damage, healing and damage reduction (not too much to make it mandatory if course) but less ressource regeneration. The day beeing the time of growth and radiance.
During the night, less damage, less healing and damage reduction but more ressource regeneration. The night beeing the time of rest and replenishment.
Thoses bonus could gradually move from a side to another during dawn and dusk instead of changing abruptly.