A sitting president cannot be indicted for anything.
Only impeachment in the House and a conviction in the Senate can remove a sitting president.
Nixon was named an “unindicted co-conspirator” during the Watergate investigation. I remember there being debate over whether a sitting President could be indicted, but I don’t know if it was ever definitively resolved. Mueller might try issuing an indictment to force Trump to have to fight it in court, unless there is a clear-cut ruling from the 1970s on the issue.