Marrok turned to dust because he was not a living person but a reanimated corpse controlled by dark magic from the Nightsisters of Dathomir. When Ahsoka struck him with her lightsaber, the green mist that escaped his body signified the magic animating him leaving, causing his body to disintegrate. This green smoke effect is a known visual element associated with the Nightsisters' dark side necromantic magic, which allows them to reanimate dead bodies as puppets or thralls. Thus, Marrok's "death" was really the release of this magic, revealing he was already dead and being used as a mystical zombie under the control of the Nightsister Morgan Elsbeth. This also explains his lack of personality and interaction before his demise, making him essentially a resurrected lapdog rather than a unique individual or a significant character in disguise.