Man seems more WTF? now than startled by the sheer absurdity (and unlikelihood) of what he’s looking at.
Grimmsy has appeared about halfway from behind the tree: enough to show that he is holding (somewhere) whatever is now close to the man. His eyes don’t appear gloating or triumphant at all; in fact, if anything they seem a bit grim, as if he’s not happy at all with what he’s doing.