Can you remove from play cards in your graveyard anytime?

The only time you can only remove from play cards from your graveyard is if a card tells you to do so.

The exact timing will be down to the effect themselves. Removing them to summon Chaos Sorcerer can only be done in your own main phases, same for removing Mezuki to activate its Ignition Effect. However cards like Necro Gardna are spell speed 2, and can be activated 'any time' within certain boundaries, ie, not in chain to a Spell Speed 3 card, and must be before or during the opponent's attack phase.