Well the thing is, Morg shield blocks multiple CC's while Sivir and Noc don't. Fizz ult works like this:
- Fish launches and attaches, slowing the target and marks the target with amplification from Fizz himself
- Shark displaces the target and deals damage
So that's 2 effects that come from Fizz ult. Sivir and Noc can only block one of these (most commonly the first CC and mark). If they block the first, the fish drops to the ground as a delayed AOE; the key is it doesn't disappear.
Morg can only block CC's as long as the shield holds, but her shield doesn't block debuffs applied with a CC effect. This is different from any other spell shield. The amplification of damage is treated as a debuff (similar to Zed ult), but it will not be blocked due to the nature of the shield. The slow will be blocked however, as will the displacement assuming the shield holds.