I think the issue is one of policy v. law. There is standing policy within the Department of Justice that prohibits indicting a President in office. However, from a legal perspective, the last time this was visited (Kenneth Starr) the opinion he had Ronald Rotunda author concluded that a President in office can, indeed, be indicted. Rotunda died recently, but revisited this subject in an editorial last year. His conclusion: Trump can be indicted, just not by Mueller due to DOJ policy.
Mueller is a tight laced, by the numbers guy. It seems unlikely he would indict Trump, particularly given the political repercussions that would ensue.
However, I think his eventual report would outline how Trump has violated xyz laws, and thereby put that hot potato in Rosenstein's lap. Rosenstein would then, presumably, provide that report to Congress. And then... All hell breaks loose.
Can the President Be Indicted? A Long-Hidden Legal Memo Says Yes - The New York Times
The president can be indicted just not by Mueller - The Washington Post