They said that you didn't know how much you loved something or someone until it was gone. Poseidon didn't think it was always true - sometimes he was not saddened but relieved, sometimes he felt indifference.
But when he saw her, looking as if nothing had changed over thousands of years, he thought that in this case, perhaps it was true. Perhaps he did miss her. Perhaps he missed the way things used to be - his ocean, her hair, their short-lived affair. Perhaps what he really missed were the younger days when he could do things without consequence - and sometimes did things because there would be no consequences. Compared to now, those days were... bright, and yet too far away.
"I see you grew your hair back." He would imagine that in the past many people were afraid to look at her directly. Even now, those who knew who she was might avert their gaze. But he wasn't like them - he thought she was beautiful, and laying eyes on her now, he didn't think she was anything less.