Inigo lets out a half-curious sound when Senku says it's fascinating. And when the other lets him look, Inigo does so. It's not quite as fascinating to him, since his brain can't really wrap itself around the implications, but at the very least he can tell that it looks really different from how Senku's did a few moments ago..

Well, at least it makes him feel a bit more reassured that he really was approved to be part of the royal family by Naga. At least enough to share her blood.

"It really is different," he just says at first. Then he finally moves back again so Senku can look at it once more. "Why is it fascinating though? Do you just like seeing how different it is?"