Can Finn choose to move up to the limit of his Speed or is it just 1 space during the Mythos phase?

he does the monster walk, and can only move 1 space.

Yeah, the (crescent) moon walk is just one space (otherwise it'd be far too powerful). His ability could have been better worded, tho. Key here is "normal moon monster", implying simply that he can move when crescent appears one space