The effect of "Mist Valley Apex Avian" which negates the effect of a Spell Card/Trap Card/Effect Monster is a Quick Effect which you activate in Chain to the effect which will be negated. This effect targets one "Mist Valley" card you control.[6]
You can activate the effect of "Mist Valley Apex Avian" in Chain to the activation of the effect of a face-up Continuous Spell/Trap Card.[7]
References
↑Konami Digital Entertainment (22 August 2014). "Putting Your Pendulum Power to Work! – Part 2". Konami. Retrieved 16 December 2015. Apex Avian lets you return “Mist Valley” cards to your hand to negate your opponent’s cards and effects, and Avian itself is a valid target for that effect.
↑Konami FAQ: When resolving the effect of "Mist Valley Apex Avian", if the card to be returned to the hand is not on the field, then does it resolve?
↑Konami FAQ: Can the effect of "Mist Valley Apex Avian" activate and target itself?
↑Konami FAQ: Can the effect of "Mist Valley Apex Avian" be activated multiple times in the same Chain?
↑Konami FAQ: [Damage Step] Can the effect of "Mist Valley Apex Avian" be activated in the Damage Step?
↑Konami FAQ: Please explain the effect of "Mist Valley Apex Avian".
↑Konami FAQ: Can the effect of "Mist Valley Apex Avian" negate the effect of a Continuous Spell/Trap Card which is already face-up?