Crab Umbra

Only Crab Umbra's controller (who is not necessarily the enchanted creature's controller) can activate its activated ability.

When Crab Umbra's activated ability resolves, it will untap the creature Crab Umbra is enchanting at that time (regardless of what creature Crab Umbra was enchanting when the ability was activated). If Crab Umbra has left the battlefield by then, the ability will untap the creature it was enchanting at the time it left the battlefield.

If a spell or ability says that it would "destroy" a creature enchanted with an Aura that has umbra armor, that spell or ability is what causes the Aura to be destroyed instead. Umbra armor doesn't destroy the Aura; rather, it changes the effects of the spell or ability. On the other hand, if a spell or ability deals lethal damage to a creature enchanted with an Aura that has umbra armor, the game rules regarding lethal damage cause the Aura to be destroyed, not that spell or ability.