That is Mike Alstott's right elbow, the one attached to the arm carrying the ball. The white line is the goal line. It's physically impossible for the ball to be across the plane of the goal line before Alstott is down, which is the moment that elbow touches the ground.
This isn't like the safety call in Denver that the refs tucked up. We don't know how that would have effected the dynamic of the game as it happened in the third quarter. This one, though, is different as all the Redskins would have had to do was to recover an onside kick, which the receiving team does some 90% of the time in the NFL.
You can talk about the other elements that cost the Redskins the game and there were many, but it came down to the Redskins needed to keep the Bucs from getting a yard. They did that but the call was blown twice, first by the official who just saw Allstott get the ball over the plane eventually, after he was down, and then by the referee who reviewed the call under the hood. It's his call. If that's no indisputable visual evidence then there is no such thing.
Here's the same picture but cropped a little wider, as some of you have requested: