Jump ship. To jump ship is to leave one company for another.
Move on. A person who moves on leaves a job. The phrase carries no blame.
Job hopper. This is someone who changes jobs often.
Poach. To poach is to hire someone away from another company.
Free agent. This is a person who is not tied to any employer.
Hired gun. This is an expert who is brought in for one job.
Golden handcuffs. These are pay or benefits that make it costly to leave.
Don’t burn your bridges. Leave on good terms, because you may need these people again.