#This is a good start, but it may even be slightly underpowered
This is a good start, but it may even be slightly underpowered
Most elven sub races give the player character multiple, small, thematic benefits. The elven subraces in the PHB all give some kind of weapon proficiency, an ability score increase, and then two minor abilities. Since the shapechange ability is relatively strong, you could have the other ability be more role play based.
One possible option would be to give the sub race something like the Forest Gnome’s “Speak with Small Beasts.” This would remain thematic, as well as still giving the player some useful ways of gathering information about the local environment.
The only other thing I would change about the sub-race is that I would add a CR cap on the Fey Shapechange ability. This cap should probably be around CR 1/8, so you still have some useful creatures, but nothing that is going to be seriously beneficial in combat.