The spell says creature that grapples you is damaged, what if you are the grappler ?
There is these different effects:
If a opponent succesfully grapples you = they succed a grapple maneveur against you --> Thorn Body deals 2d6+CL damage to them.
You initiate grapple, or takes control of a grapple --> Thorn Body does nothing.
You maintain a grapple, and choose to deal damage --> Thorn Body adds 1d6 to the damage, IF you deal damage with an unarmed or natural attack.