Zed's ultimate will still make Zed briefly untargetable while he dashes to his target's location, and Zed's living shadows will behave as normal. However, the mark itself will cause no damage if it is removed in the 3 seconds before it detonates.
Remember that Quicksilver Sash does not give you any lingering resistance after use like Cleanse. If you use it to remove a slow/stun/etc, you can immediately be stunned, slowed, suppressed, etc. again.
The active is one of the few abilities that can be activated during silence or suppression.
Quicksilver Sash's name is derived from the chemical element mercury, which is also known as quicksilver.