Encroaching Mycosynth's ability causes Auras to become artifacts. This makes it more likely that another effect (such as that of March of the Machines) will make those artifact Auras into creatures. An Aura that's also a creature can't enchant anything. It's unattached the next time state-based actions are checked, and then immediately put into its owner's graveyard as a second state-based action.

Encroaching Mycosynth turns all permanents you control into artifacts. Permanent spells you control become artifact spells in addition to their other types, and nonland permanent cards you own in other zones become artifact cards in addition to their other types.