The answer: Cassie is wearing a white hat.
Alex did not know what color of hat he was wearing. This rules out the option of seeing two black hats because then he would know his hat was white. This means that he either saw two white hats or a black and white hat combination.
Even with Alex's information and seeing Cassie's hat, Bobby did not what color of hat he was wearing. Using all of these clues, Cassie knows that if Bobby saw a black hat, he must have known his hat was white. Since he did not know the color of his hat, that means he saw a white hat.