Does hidden investigators still have to evade monsters before move? Asking about dexter's spell specifically
Hidden Investigators must unhide as an action before moving. You mention "Dexter's spell" specifically. The only spell he starts with is the Mists of Releh spell which causes an investigator to be considered hiding.
Unless you were speaking of Dexter's once-per-game ability, Disappearing Act. For that, no, you do not need to make an evade check.