Effects like the one generated by Argent Mutation, Tezzeret, Agent of Bolas or Inkmoth Nexus all give a permanent an additional permanent type. It is important to note that these effects only apply while the affected permanent is on the battlefield. Once it leaves play, any of these effects will no longer apply to it.

For example, if an Inkmoth Nexus is animated into an artifact creature and is dealt a point of damage, then this card will go to the graveyard. While in a graveyard, it will only be a land card. It cannot be targeted by something like Buried Ruin‘s ability or be counted by Slag Fiend‘s ability. It does not matter that it left play as an artifact creature land; when it left play, the effect that made it into an artifact creature stopped applying to it. In any other zone, Inkmoth Nexus will just be a land card.