Dress down didnt stop magus of the moon
I had dress down on battlefield. Opponent had magus of the moon. My tundra could not give white mana. Why? Magus is stopped, no abilities.
Or is it no bug and magis still works under dress down?
This is not a bug, but a result of mtg's layers system. Magus of the moon (and blood moon, for that matter) apply a type-changing effect, which is applied before the ability-losing text of dress down, regardless of timestamps. So yes, magus still works under dress down.