What should I do if my master's advisor used a figure from my thesis in a publication without citation or acknowledgment?
The issue here is somewhat thorny. Relative to you, however, there is unfortunately not much benefit in any event, because citing the figure is not normally enough to merit authorship. All you would have is one extra citation, which would likely get "lost," because the publication with which it corresponds doesn't exist yet.

So I would make sure that your new paper cites the old paper, since it is the first to publish the figure. I would also make sure that you mention politely the concerns that you have over the use of the figure without citing.