This episode was pure artgasm, and... Berserkgasm maybe?
For me, the transparent branches and the moonlight boy standing on them was the best, this detail and realism with just black-and-white screentones is simply incredible.I doubt that the boy is Griffith himself spying on them, but maybe he's his son, too? Because I'm starting to think that Griffith didn't only "demonize" the baby during the rape, but actually added a bit of himself to it, becoming a third parent somehow (probably not by purpose). Since he was already belonging to another dimension during the intercourse, the resemblance to him only appears in the boy's astral form, while his physical body is a mix of Casca and Guts. Maybe, that's why the boy now seems to follow Griffith's orders or something, because Griffith is actually his other father? Maybe Griffith only realised that it's his son after incarnating, so he gave him a body and manipulating him somehow.
Now, really just speculating:
I think the only thing the boy wants is to be with his parents, that's his instict, but he has a hard time deciding which parents should he join, Guts&Casca, or Griffith, because both offer him something - Griffith gave him a normal physical body, but Guts and Casca can give him a family - and that's why he seems to seek for Casca's company but be related to/influenced by Griffith at the same time. (ok I think maybe I'm getting too far

)
I'm not sure if an idea like this came up before but at the moment it seems logical to me.