"creep" literally means move stealthily, and often quietly, along a surface, like when a spider moves along a wall, or when a cat is hunting rats. "mission creep" is when a mission expands slowly beyond its initial scope, and possibly exceeds its permitted limits. It's often used in the case of military interventions. The UN may authorise a military intervention on very strict terms, but on the ground, the situation gets complex and the troops find it difficult to restrict themselves to their precise mission.

The expression is 'mission creep'. Here's the wikipedia definition: Mission creep is the expansion of a project or mission beyond its original goals, often after initial successes. Mission creep is usually considered undesirable due to the dangerous path of each success breeding more ambitious attempts, only stopping when a final, often catastrophic, failure occurs.