Board Thread:Episode Discussions/@comment-1181222-20180328101515/@comment-25132842-20180401150751

Vladosaurus wrote: How did amaterasu not burn that white Zetsu to the ground to begin with? Also it is pretty much hinted that Mitsuki is related to white zetsu. And since white zetsu has a lot of life force, it would also explain how he is able to use Sage Mode.

Mitsuki can use Sage Mode because he has Jugo's Kekkei Genkai. His ability to use Sage Mode is entirely subconscious. That's why at first, he couldn't fully control it and only went into it in times of great stress.