The impression I got was that they're targeting his campaign overall more than him specifically, and to me it seems much more plausible that some campaign members were conspiring with Russia or other foreign agents than just Trump himself.
I'm still doubtful it'll go anywhere.
At least... 4? I haven't been counting, but I know at least 4, Trump associates have either been found guilty of, or signed a confession to, some kind of crime by Mueller. Mueller has also been continuously seeking more and more info from Trump, for example looking into various aspects of his finances. Mueller has already indicated that if he finds something outside his scope he will tip off another agency (for example the tip he gave on Michael Cohen, Trump's laywer that resulted in all of his files being seized in a morning raid by FBI); so the fact that he's gathering info on Trump means that he thinks that info is within the scope of his investigation. Which is about, among other things, collusion.
This is ignoring all of the huge red flags have not yet resulted in overt legal action. Donald Trump Jr's ridiculous emails where he said he was "enthusiastic" about colluding with Russia. Trump during the election openly stating on live TV that if the Russians had "the emails", they should release them. Trump claiming he doesn't know who Papadapolous (one of the middlemen between Russia and the Trump campaign, who got caught with pretty damning evidence and agreed to cooperate) despite him being a high up position in the campaign. Then a picture comes out with, oops, Papadapolous in the same meeting with Trump. Also that meeting between Donald Trump Jr and a Russian lawyer that's basically a legal kiss of death thanks to his emails? That he claimed only he was there for? The majority of Trump's higher ups in his campaign were there and "forgot" to mention it. Also, there's that campaign staffer that got subpeoned for emails by Mueller and had a 6 hour long mental breakdown on live TV. When he was asked by a (I think, CNN) anchor if he "thinks Trump actually did anything", his response was "...all I'm going to say is, I think Trump mayyyy have done something really bad. And I think Mueller might get him for it." Obviously not legally significant but, you know. Doesn't look good from the outside.