Is his use of the Giant symbol optional?

Please quote what you ask about so we can give an accurate answer.

Dark Lord does not allow him to "use" the giant damage symbol. It causes him to possess it. If a character could "use" the giant damage symbol via a special power, it has the option to use the giant damage symbol on an action-by-action basis without canceling the power. Since the power causes Mordru to possess the giant damage symbol, you have to cancel the Dark Lord power in order to not use it. If you cancel Dark Lord then you won't be able to use any of the powers that it lets you use either. The power stays canceled until the end of the turn.

So it is effectively optional, but you have to sacrifice everything else that Dark Lord lets him use in order to not use the giant damage symbol.

However you can use the peek-a-boo tactic with Dark Lord special power. You can be a giant and shoot over figures, and on your opponent's turn you can cancel the power and return to a normal size, and thus become a smaller target.

This is what I was asking. I guess I didn't phrase my question correctly.

I believe that this would cause his Smoke Cloud to disappear, in the unlikely event that giant Mordru couldn't come up with anything better to do than generate a smoke cloud.

Actually, canceling a power does not stop the effects already in play. If I throw up a Smoke Cloud and then cancel that power, the smoke remains.

Along these lines can Skrull Yellowjacket use the peek-a-boo tactic also.

yes he can