If my previous employer gave someone a copy of my SS card without my consent, what can I do?

Unless and until something detrimental happens resulting from this disclosure, you have no recourse. In other words, until you suffer a harm that results from your empoyer's neglience, you have no claim. And even then you would have to prove causation; that is that this action caused your loss.

You can't do anything until and unless something happens, other than possibly asking the employer to pay for one of those "life lock" or credit monitoring/protection services to guard against identity theft or fraud--but it is voluntary for the employer to do this, so while you can ask, you cannot compel them to. The problem is, while the risk from an improper release of your SSN or SS card is clear, the law does not provide compensation or remedies for "hypothetical" injuries or losses--what could happen, but has not. IF you are victimized by identity theft, fraud, etc., then you could sue your employer for the costs, losses, etc. you suffer with a reasonable chance of winning if you can in any way trace the act to the release of your card (e.g. your SSN was used, or an impage of the card; the perpetrator is this former employee or someone connected to him; etc.). But hopefully nothing will occur; and if nothing occurs, you cannot sue.
(You may, of course, as a protective measure, choose to pay for one of those credit monitoring, etc. services yourself.)