Depends on what you (or these aliens) call slavery.
We've used many definitions over the years. "Work without pay" is the Sunday school definition, but in all cases slaves are compensated with room and board,
at least (otherwise they would die of starvation in about 7 days), and sometimes education and extra money for nice things.
In indentured servitude, the slave had a somewhat ambiguous time for when they would buy themselves out of the contract. In both indentured servitude and Dutch/American slavery the "owner" had considerable control over decision making - where they could work, who they could socialize with. Discipline could take any form from cutting extra benefits, reducing room and board, or beating and imprisonment.
In "company towns" where people had no practical alternative to working for some local employer the same powers and discipline applied. Locals, on paper, had freedom to petition their government for grievances, but it was a dead-end practically speaking.
Likewise, in feudalism, serfs could petition the ruler (higher rank baron/baroness or the king/queen) if a local lord was abusing his custom, or badly mismanaging the land. The lord's powers were thus limited by whatever protections people had carved out for themselves, provided the higher level of governments wished to enforce those rules.
In our modern system, an employee may leave an employer - blackballing is a possibility. Some employers are demanding more visibility into what an employee does off the clock and who he/she socializes with, and they discipline employees acting in a way the employer sees as unbecoming. On paper, an employee may move to any employer he or she wishes, or choose no employer at all. But larger economic forces, such as unemployment rates, can drive skilled professionals into minimum wage, abusive jobs because there is "nowhere else to go".
If you choose the last of these systems (and your aliens are certainly smart enough to), you can encourage people to produce a lot, with less than room and board in compensation (people tripling up in an apartment to afford rent). As long as people genuinely believe that they must stay plugged into the system and that one day it will pay off, you keep things under control without the need for any great spending in armed force. In this case, though, the people aren't bitter. They consider themselves to be struggling against forces of nature or fate or mathematics, not against aliens.