The legal flex is that contractually all work during work times is the company's IP. While the library I built has helped our company it isn't our product and it isn't my job. What I do for a salary is build our product not build this library. On a human moral level I would personally give my subordinate the right to open source the library because it was purely his creation.

As for what I could be subject to, could this be considered discussing publicly private company information?