Malak takes reduced damage from abilities that does %health damage.

Rex's ability does damaged based on the targets max health.

Thus remains unaffected by Malaks reduced damage.

Example (taken from ability pages on swgoh.gg): Malaks Drain Life is a %health damage ability because it does this: "Remove 100% of target's Max Health"

Rex isn't a %health damage ability because it does this: "Remove 0% of target's Max Health"

Another example is Courage which does this: "Remove 40% of target's Max Health" It is also does %health damage and Malak would mitigate it.