Is this a scam / phishing pop-up -- or a real concern?
The answer is "it depends."
Password Monitoring is a legitimate Apple service. Read about it here: Password Monitoring – Apple Support. However, I do not know if your original post included a screenshot of that alert or not because it has been removed.
I do not know the reason it was removed, but if it included confidential information, that alone would give me pause to wonder if you have been less than circumspect with your personal information. And — if that is the case — I suspect the warning is legitimate.
In an abundance of caution, you should consider changing the password associated with your Apple ID according to these instructions: Change your Apple ID password - Apple Support. Do not use any other instructions, and don't interact with whatever website provided the warning you described. Unless proven otherwise assume it's a scam.
Then — even though it has been archived — review What to do after you change your Apple ID or password. Use that document as a general checklist to review all the Apple products and services that rely upon authentication.