Hello, I am currently working in a government contractor company (in
the United States), and I recently got cleared for a security
clearance.
I'm trying to move to another govt contractor company for another
career opportunity. But the prospective employer is asking me to
provide them a list of which government clients I've worked for in the
past (including the current employer), so that they could cross-check
if I'm legally okay to work with them.
Having personal direct experience with getting and maintaining a security clearance in a previous life, it seems strange they are asking for a list of your employer's clients, that seems something that would definitely be considered confidential. At the end of the day, you are in the best position, to determine if that list is confidential. Furthermore, it makes a difference, if this information has been classified Confidential or it's simply confidential business secrets.
(There is a non-solicitation and confidentiality agreement that I've
signed with my current company)
You will have to read the confidentiality agreement to determine what information it covers. The non-solicitation clause, exists to prevent you from trying to get your coworkers to leave the company together, and taking your inside knowledge with you.
However, even if you leave the company, since you are dealing with classified information, certain restrictions will remain. You will be unable to share that information with anyone who doesn't have a need to know. Additionally, you will still be beholden to the confidentiality clause, which is exclusive and inclusive of any secret information you might know. I also assume, you signed a NDA, for any government information you have been made aware of. Be aware that NDA would be separate from any confidentiality clause with your company.
if you have any question about either clause I would contact a lawyer. However, I must warn you have something, once you leave your current employer your security clearance will be temporarily suspended. What this means, while your security clearance is in this state, you will not be allowed access to classified information.
They also asked me to double check the list with my current manager to make sure, so that I get the correct list of clients. (which makes it feel sketchy, and makes me feel I'm going behind my current employer's back)
You need to determine the security classification of the list in question.
Is this type of process normal/typical thing to do when moving from a govt contractor company to another? If not, what should I do?
When I had a clearance, I never was asked, for a list of agencies that I had worked with. In some cases, I wouldn't have been able to name the agency, at least not on any system my resume would be viewed on.
And in terms of providing them a list, if I knew what client/project that I was supporting for even when I was uncleared, does that mean that my client/project is not cleared/confidential?
If the information is classified as Confidential then you will be unable to release that information to anyone without a need to know. However, information can be classified as Confidential, from the perspective of the U.S. Government. However, a company can also have confidential information, there is a huge difference.
Providing Confidential information on an open system, to somebody that doesn't have the need to know that information, will result in you losing your own security clearance.