I just discovered that one of my employees works on the weekends waiting tables. Should I be concerned?
Employee side hustles are common. According to a survey of 1000 adults, 93 percent of those surveyed had another job in addition to their full- or part-time job. Most who do extra work do so for something to do, but also to earn more money. https://www.benefitnews.com/news/insuranks-survey-on-side-hustles-reveals-what-drives-workers
So, if you were to prevent side hustles, you may lose employees and candidates.
For example, if Tom works as a salesman during the week, but renovates and flips old houses with his son on the weekend, that is a side hustle. To tell Tom he can no longer flip homes with his family would undoubtedly make Tom ask "why not", and lead him to ignore the request or find a job that allows him to do so.
However, if Tom works for his employer's main competitor on the weekends, that is a problem, especially if Tom uses sales contacts from his main job.
The takeaway is that if the employee in question is performing well and does not create a risk of losing confidential information or putting others at risk, you should consider allowing employees to do other work. Your concern is to make sure that the side hustle does not undermine your organization's ability to meet its mission and the employee's ability to perform their job duties.