The other answers highlighting your need for legal advice right now are correct, but for reasons other than they give.
As the employee disclosed no code, copyright is not a concern here - in pretty much all jurisdictions, you cannot copyright an algorithm.
Patents are an approach, but again have different issues in different jurisdictions - some jurisdictions are "first to file" and some are "first to discover", so you may not have any retroactive cover from any patent applied for today, even assuming the algorithm is patentable.
What you can attempt to pursue is protection for your algorithm under trade secrets laws if they exist in your jurisdiction, which can apply protection and civil and (often) criminal penalties for unauthorised disclosure, even when no copyright or patent is violated.
Also check your employment contract for any non-disclosure agreement, which would certainly apply in this case. Seek legal advice as to how enforceable it is in your jurisdiction and in this specific case.
Once it's been established what right of control you have over the algorithms disclosure, I would suggest you have a full team meeting with everyone who has access to the code base in which you discuss what happened, why it is inappropriate to disclose such things as they can be extremely detrimental to the companies operation and their continued employment, and also highlight the potential civil or criminal liabilities such actions can result in.
If the specific employee in question does not come forward after the team meeting, there is little you can do without proof of identity - you can of course hire an investigator to see if they can uncover proof, but then all of this really comes down to what course of action are you intending to take if you do discover their identity.
A slap on the wrist and a "that was really damaging to the company, please don't do it again"? - well, the team meeting will have covered that.
Firing the employee? Intent on taking legal action against the employee? - depends on how badly you want retribution.