Roy's eyes lit up. Alexstrasza's seemingly venting words gave him a lot of information.
First of all, Alexstrasza knew him. Otherwise, she wouldn't have shouted his name the moment they met. But it was still unknown when she met him.
The second piece of information was about Nozdormu. Since Alexstrasza knew about Roy, Nozdormu naturally knew him too, and even Malygos knew. If this guy hadn't been in such a muddle-headed mental state, he might have recognized Roy at first glance.
In other words, all the Aspects had probably interacted with Roy at some point.
As an outsider, Roy had interfered with the normal history of this world as soon as he entered it. He knew this long ago, and he also knew very well that Nozdormu, the Guardian of Time, would spare no effort to correct this history.