Again, no he doesn't. All Mueller needs is to be lawfully within the scope of his original investigation and it's fine if he finds evidence of other criminal activity.
Look at it like a search warrant. The warrant needs to be lawful and limited in scope. So you can't from the get go exceed the boundaries of that warrant. But if you do find evidence of different illegal activity, you can still legally obtain it. So if I wanted a search warrant for your house because I had probable cause to think you committed a bank robbery, I would lay out my case probable cause for what I expected to find at your house relating to a bank robbery. So lets say, I can lay out a case that I'm likely to find the gun used in the robbery at your home. Then, while conducting my lawful search of your home, I find a suitcase full of cocaine. I wouldn't be obligated to disregard the drugs. They would have been found during a lawful search.
What I couldn't do, and you're right here in regards to scope, is in the same scenario where I only had a search warrant for your house, would be to go search your buddies house across town. That would be outside the scope of my legal grants and any evidence would be thrown out.
Just because Muller found something (somewhat) unrelated to coordination with russians, does not make this unlawful. The argument hinges on whether or not it was within the scope of his prosecutorial powers to be looking into members of the trump campaigns finances when he found this evidence in regards to another matter. And looking into finances to see if there was any coordination, was certainly within that scope.