I’m wondering if one were to take a light colored dirt and mix with a little water to make a mud if it could hide that gap a bit? Probably take some experimenting to get color right but once it dries it may improve the appearance. Just a thought.
I don’t think that’ll help much, to be honest. The entire detachable hair piece looks like a wad of flattened, chewed on Play-Do just draped over his head. The rest of it looks amazing, but that hair is a serious let-down for me when I know they are capable of better.