I don't understand how he's still open
It's because you can't just stick a strong perimeter defender on Green and lock him down like you would a normal 3-point specialist who's shooting out of his mind; you can't do that to any Spurs player, really.
The Spurs offense is predicated on constant curl screens/P&R/offball movement. There's really no effective way for Miami to guard it. Either they switch on P&Rs/screens and end up with terrible matchups or they stick to their man chasing them through multiple screens and wear themselves out throughout the game (You've been able to really see it with LeBron this series, by the 4th quarter that nikka's sucking air and hardly having an effect on the game because he's EXHAUSTED from chasing Green/Parker/Neal etc. through all kinds of double/triple screens every possession).
Even if they were to dedicate themselves to shutting Green down with doubles, that frees up Manu/Neal/Parker/Leonard on the perimeter and starts giving them open looks and driving lanes that they didn't have before.
It's really a pick your poison type deal. The Spurs motion offense is too advanced and Green's too good at setting himself up off the ball for Miami to just key in on and him and shut him down without causing a world of other issues and mismatches on the court.